knowing nothing about browsers I'd guess that's not great but wow that's an impressive number
Computers are faster when they have more transistors, so I don't see why having more extensions wouldn't make a browser faster.
my (very vague) understanding is that browser extensions are a growing attack vector these days, which is why i've always been reluctant to install them
oh absolutely there is a bunch of malware in extensions! i mostly use open-source ones, and if not they have to be really big like BTTV or 7TV but you still gotta be careful with that