Grep is inefficient search engine, because it needs to scan through whole content (and Telegram uses search indexes). Also, grep cannot deal with words forms and inflections (you type "foot" and you also want to find "feet"). Inflections are not very important in English, but you need to deal with them in other languages where the word can have many forms.
I'm not trying to claim Telegram uses grep or whatever. My point is even very active chats on telegram generate somewhat small amount of text data and I dont believe that searching through it require massive complex search engine with super-fast backend.
I basically participate in hundreds of chats and message history doesn't take 10 of GBs. And I also know that search in history of such chats isn't so snappy on older Android phone.
