Reddit mentions of StarTech.com Dual 4K Thunderbolt 3 Docking Station - Windows & Mac - 15W Power Delivery DisplayPort & TB3 Laptop Dock (TB3DKDPMAW), Gray, No Charging | 2X USB | 1x DP, TB3

Let's go back in time to VGA / DVI! These were great for 1,920*1,200 @ 60 Hz (3.96 Gbit/s). As bandwidth demands grow, Dual Link DVI (2560 × 1600 @ 60 Hz) gets used mostly for 30" screens but since it needs to more pins it doesn't become popular.

Then came DisplayPort 1.1 which had the rough bandwidth equivalent of double of DL DVI, thus it was capable of driving a 4K monitor at 30 Hz. HDMI was originally the same (video wise) as single link DVI, later it got its clock speed doubled and this became a widespread standard so much that most DisplayPort sources are in fact so called DisplayPort++ which are capable of emitting a HDMI signal. When you see passive, cheap DisplayPort-to-HDMI converters, this is why: it's only a plug converter, the signal coming out of the DisplayPort socket is HDMI.

Then came DisplayPort 1.2 which doubled the bandwidth again, thus being able to drive 4K at 60 Hz. It also had the capability to drive multiple monitors from a single connector by feeding that signal into what's called an MST hub. DisplayPort monitors offering daisy chaining contain such an MST hub. Here's how much bandwidth typical resolutions need:

> 1920x1080 (High-definition 1080p) @60hz - 22%
> 1920x1200 @60hz - 30%
> 2560x1440 @60hz - 35%
> 2560x1600 @60hz - 38%
> 3840 x 2160 (Ultra HD 4K) @30hz - 38%

Mix and match to your hearts content up to 100%, four 1080p @ 60Hz etc. But three 2560x1440@60hz won't work, you can't say "it's just a little over 100%".

Then came USB C which is a gigantic mess. It has four high speed links which are able to carry different signals, one of them is a DisplayPort signal. If you get a cheap USB C to DisplayPort device, that's what happens. USB C also contains an USB 2.0 connection which is available while using DisplayPort. Some adapters (Xiaomi has a great one) will give you a DP port and some USB ports.

The same four lanes are capable of running high speed USB, up to 10 gbit/s and also the USB 2.0 connection. It is possible that some USB C docks will have USB 3.0 marked ports which run at 3.0 speed (5gbit/s) when there's no DP cable plugged in and fall back to 2.0 when you plug in the DP cable. Because Macbooks only have USB C lately, adapters and hubs became popular for them. You will notice some have two USB C plugs -- no wonder, they use the high speed link in one for DisplayPort and then they need the other for speedier USB.

Another way to use the four high speed links is to carry Thunderbolt 3. Thunderbolt 3 is a 40 gbit/s bus which contains a mix of various signals, for example, two DisplayPort signals and some PCI Express as well. Since today all panels are of the Embedded DisplayPort variety you need a thing which takes the DP signals off the TB bus. This can be inside the monitor, that's what a Thunderbolt monitor is. 5K monitors with a Thunderbolt connection will happily take two DP signals off the bus to have enough bandwidth (and/or have two DisplayPort inputs). 4K @ 60 Hz monitors (and lower) will take only one signal down. A dock also can take two DP signals off or one DP signal off. To complicate matters, the one DP signal can be fed into an MST hub and so it's possible you are looking at two digital outputs and have no idea how much bandwidth you got. The manual tells it all: if the resolution table says one monitor can be 4K @ 60 Hz but two is 4k @ 30 Hz, that's an MST hub, splitting the bandwidth.

So your options are:

1. If you want to run a single monitor up to 4K and low speed peripherals, look no further than the Xiaomi USB-C to Mini DisplayPort Multi-function Adapter. 30-40 USD. Add a two port MST hub to taste for another 30-ish.
   
   
2. If you need more video bandwidth but no USB peripherals, use the Plugable adapter. 70-ish USD.
   
   
3. If you also need speedier peripherals, you need to buy some sort of Thunderbolt device. StarTech has a cheap single DP dock which also provides a built in USB 3.0 with a few USB ports visible (the Xiaomi USB 3 looking ports are 2.0 speed! there isn't a way to do otherwise) and a couple devices (Ethernet, audio) hardwired to the other ports of the hub. Lenovo's dock is a terrible deal, really because takes only one DP and contains an MST hub while the Dell TB16 is less than 10% more expensive but demuxes two DP. Single DP 200, dual DP 300-ish USD.
   
   
4. If you need speedier peripherals not provided by the docks, get the Akitio Node Lite and use a PCI Express card to have at it. Note I recommended this one because it also demuxes a DP port for your convenience. 270-ish USD plus whatever card you will want. There are smaller, dedicated devices for most purposes in this category (10 gigabit ethernet, video capture etc) but they usually don't provide a DisplayPort and hideously expensive.
   
   
5. If you need a lot of video bandwidth and/or graphics performance, you can buy a Thunderbolt external GPU chassis which allows you to plug in a desktop PCI Express graphics card. Most consumer cards will give you only three DisplayPorts but more professional cards will give you 4-8. NVS 810 and four 5K monitors, yummy! :D The reason I emphasize DisplayPorts so much should be clear by now: they stand in perfectly for HDMI 1.4 and also can be split with MST hubs. You don't necessarily need to bring your own videocard: Gigabyte has one with a built in short 1070 or 1080 and built in PSU and USB port, Sonnet has a puck which is relatively small but provides no USB ports, Galax has an upcoming 1060 in a similar physical format. Note the Node Lite can be hacked with an eGPU too. https://egpu.io/ is your friend. 400-ish USD and up, up, up.
   
   Real big confusion points include:
   
   
6. many laptops have a half Thunderbolt implementation with only one DP signal on the bus. This is never documented anywhere, it just comes from user reports. There's a mountain of confusion when one person says "this has only one DP signal" and the other says "but I am running two monitors from it" because the latter has an MST containing dock like Lenovo's...
   
   
7. You never know what an USB C connector can provide. You plug a cable in it, you plug it into another device , it doesn't work, you can begin guessing: maybe the cable is not in spec, maybe it's in spec but the wrong spec (a 10gbps USB C cable won't work with 40 gbps TB3), maybe your host dislikes the controller of the device (google macbook ti82), maybe the device wants the USB C to use its four high speed lanes in a mode that the host is incapable of and any other number of issues
   
   Ps. HDMI 2.0 is a late comer, many years later than DP 1.2 and it has roughly the same bandwidth as DP 1.2 but as it is a late comer, DisplayPort++ doesn't apply and you need an active converter. You need to read manuals carefully to see what the HDMI port gives you, as many docks and converters won't do HDMI 2.0 they will say "4K" but that's just "4K @ 30 Hz" and not HDMI 2.0 which is required for "4K @ 60z". All in all, venturing into anything above the usual low bandwidth resolutions you need to read all manuals carefully. Acer has a new monitor which, on the cusp of the demise of DVI uses DL DVI to drive a monitor at 144 Hz. Anyone sane would've used a DisplayPort.

This hub has more than one Thunderbolt 3 / USB-C port. It's also insanely expensive.

I think it’s a power or throughput issue. The only time I’ve seen a USB-C hub that has more than one USB-C port always have two USB-C connectors back to the MacBook Pro.

Aluminum Thunderbolt 3 USB Type-C 2016/2017 MacBook Pro Hub Adapter Dongle with HDMI. 1-year Warranty. 40Gbps TB3, 4K HDMI Video Output, USB-C, microSD/SD Card Reader, 2x USB 3.0(Space Gray) https://www.amazon.com/dp/B075ZTQ895/ref=cm_sw_r_cp_api_Kd2IAb8RA23ZD

USB C Hub, Nicetech NT-10 USB C hub 3.1 with Type C Charging Port, 40Gbs Thunderbolt 3, 4K HDMI Video Output, SD TF Card Readers, 2 USB 3.0 Ports for MacBook Pro 2015/2016, USB C hub now in Space Grey https://www.amazon.com/dp/B076Z2C8T6/ref=cm_sw_r_cp_api_ue2IAbDW4P81P

I did find this one hub, which may work. Mixed or insufficient reviews:

USB C Hub,NIGI USB C Multiport Adapter 3.1 with Power Delivery Type C Charging Port ,Micro sd Card Reader,2 USB 3.0 Port,1 USB C Port for MacBook Pro 2015/2016, Google Chromebook 2016 - Silver https://www.amazon.com/dp/B01MY96CWH/ref=cm_sw_r_cp_api_7e2IAb2QZV6XH

You could always try the reverse with this adapter, if the device doesn’t require too much power or speed:

USB C to USB Male Adapter, Acessorz Mini Hi-speed USB C Female (Type-C) to USB A Male (Type-A) Fast Charging Sync Connector Converter OTG Adapter for Laptop / Wall Car Charger with USB A Interface https://www.amazon.com/dp/B075LHXVNV/ref=cm_sw_r_cp_api_mh2IAbRB8Y075

Then there’s always the big guns:

Wavlink Aluminum USB 3.0 Universal Laptop Docking Station & Vertical USB-C Display Dock Dual Video Display HDMI/DVI/VGA, Gigabit Ethernet, 2 USB-C Ports, 4 USB 3.0 Ports, Audio Supports Windows & Mac https://www.amazon.com/dp/B01M7PP31E/ref=cm_sw_r_cp_api_6n2IAbTR3534X

Wavlink USB 3.0 & USB C Ultra HD/5K Universal Docking Station, Dual 4K Dual Video Monitor Display with 2xDisplay Port, 2xHDMI, Gigabit Ethernet, Audio out and Mic in,4 USB 3.0 Port, 2 Type C https://www.amazon.com/dp/B076BG9YVQ/ref=cm_sw_r_cp_api_ek2IAbQW6VHXF

StarTech.com TB3DKDPMAW Thunderbolt 3 Dock - 4K - Laptop Docking Station - Thunderbolt to USB-C / USB 3.0 / Gb Ethernet - Mac / Windows https://www.amazon.com/dp/B01MSEKBNM/ref=cm_sw_r_cp_api_Yk2IAbFJX2S8X

For whatever it's worth, I have this dock plugged to a 4k monitor and it drives it at 60hz no matter which port it's plugged into.