Sadly, judging from FF and Chrome both, extensions lead to unavoidable slowdowns. Which makes them a "can't live with them, can't live without them" proposition. Every time I create a new profile, I am shocked by how fast it is. Then the avalanche of ads and tracking pixels forces me to install ABE or uBlock, and the downward spiral begins.
Definitely, that's probably one of the biggest dangers of opening IE up to more extensions. On Windows at least, the divide between Chrome/FF and IE is more fundamental though. Last month, I spent a week really seriously trying to evaluate switching to Firefox or IE, using equivalent extensions (essentially just LastPass, IIRC). Chrome was still awfully clunky compared to Win10's IE TP, especially around touch scroll/zoom/swipe inputs.
