I know of no such research, however I think that the actual question may not be readability, but the effects of improper usage on the credibility of the author.

It may not be fair, but poor grammar and syntax reflect poorly on the author. It represents a "stumble" that takes the reader out of the message by forcing attention to the improper usage.

I realize that in text messaging and some other very-low-level consumer applications, people do not always capitalize, but any professional-level writing must comply with the rules in order to be effective.

Michael W. Marek, thank you for your answer, but I am in fact looking for readability and/or orthographic processing studies about the initial capital letter at the beginning of a sentence. This hasn't got anything to do with poor grammar - these are deliberate choices where a brand's name (iPhone, eBay, adidas etc.) or personal names (k. d. lang, danah boyd, will.i.am) are recognized and in a way "respected". The same is true for certain abbreviations, such as pH, rDNA, t-test etc.

The first article targets speed and maybe provide at least some glimpse at a direct effect.

Babayigit, Ö. (2019). The Reading Speed of Elementary School Students on the All Text Written with Capital and Lowercase Letters. Universal Journal of Educational Research, 7(2), 371-380.

I agree with Mark wholeheartedly in most contexts, but intentional branding and advertising initiatives may be special cases where the rules are a bit murky. I am not sure I could commit 100% to this second article but it does tie effects to the intentional use of lowercase and ultimate investment intentions.

Song, Y., Luximon, Y., & Luo, J. (2020). A moderated mediation analysis of the effect of lettering case and color temperature on trustworthiness perceptions and investment decisions. International Journal of Bank Marketing.

Hope this helps. -jd

J. D. Wallace, thank you! I'll check them out - the articles look promising!