Reddit mentions of Dual Language Development & Disorders: A Handbook on Bilingualism & Second Language Learning, Second Edition (CLI)

I'll go against the grain here. Modern consensus in the field of language acquisition research is that there is no causal relationship between multilingual upbringings and speech delays.

The go-to book here is Dual Language Development and Disorders by Genesee, Paradis, and Crago (eds., 2011). It's very detailed and scientifically sound without being too alienating to non-specialists. Most chapters are written in a Q&A format. Chapter 3 is most relevant here, so I'll draw some quotes, emphases mine:

>Starting with the early stages of verbal development [...] Oller and his colleagues examined 73 infants (monolingual and bilingual) longitudinally from 4 months of age to 1.5 years of age. They found that the onset of canonical babbling for the bilingual infants did not differ significantly from that of the monological infants [...] and emerged at the same time (Oller, Eilers, Urbano, & Cobo-Lewis, 1997). Similarly, Maneva and Genesee (2002) reported that French-English bilingual infants exhibited [babbling] that resembled that of monolingual French and monolingual English infants of the same age.

>Studies that have examined lexical development in infants and toddler report the following: bilingual children produce their first words at about the same age as monolingual children, 12-13 months on average (Genesse, 2003; Patterson & Pearson, 2004); bilingual children's rates of early vocabulary acquisition generally fall within the range reported for same-age monolinguals, as long as both languages are considered together (Conboy & Thal, 2006; Pearson, Fernandez, & Oller, 1993)[...]. Petitto and her colleauges reported that first words, first two-word combinations, and the acquisition of the first 50 words occurred within the same age range... (Petitto et al., 2001).

>Finally, longitudinal studies [...] found that early word combinations emerged within the same general timeframe as that found for monolinguals, which is approximately between 1.5 and 2 years of age (Padilla & Liebman, 1975; Paradis & Genesee, 1996). More recent research with larger sample sizes has also shown that [bilingual children] begin to produce word combinations in both or at least one of their languages in the same age range as monolinguals (Conboy & Thal, 2006; Marchman et al., 2004).

The idea that a multilingual upbringing leads to delays is an old myth that lingers on despite not being grounded in scientific evidence. I remember encountering this myth in Ling 101 and a subsequent undergrad course on language acquisition, and no one pointed me to the evidence to the contrary until I was in a grad school program specializing in multilingual acquisition.

It's possible there are speech delays at work here that have nothing to do with the multilingual environment. Unless your child was separated from an identical twin at birth who's now being raised monolingually, it's hard to know or compare, but the studies that we do have don't point to causality or even plain correlation.

TL;DR: Modern evidence shows no significant time scale differences between monolingual and multilingual kids, but old ideas die hard.

(Since you asked for professionals in the field: am linguist, trilingual parent of a trilingual kid, MA, language teacher, etc.)