Pretty soon afterwards I realised my browser just felt slow, so I looked into what was going on. Turns out metamask pulls in web3.js, which alone is 2.3mb of javascript. Simply parsing that much code in each chrome tab was taking hundreds of milliseconds per page load. Web3.js is so ineptly crafted. Some of its dependencies get bundled a lot of times at slightly different point releases, for no forseeable reason. I opened an issue about this years ago, and despite lots of attention and promises it still hasn't been fixed[1].
The bigger issue with metamask is that (at least at the time) it allowed any site you visit to silently query your crypto wallet address. Talk about a privacy invasion. Every website you visit gets an ID that I'll keep across all my devices, through which you can find out exactly how much money I have, in what cryptocurrencies. And they can use that to track my purchases. Forever. I raised this with the developers and they said "oh yeah we know about that and we'll fix it later". (Edit: sounds like this security problem was eventually fixed months or years later.)
What on earth? People want to build the future of the web on this junk pile? Holy cow this is a house of cards.