Definitely flash, especially on Linux, just moving it to 'click to play' is making the browser much much faster.
I think it's 'click to play' by default now.
No, I have Flash disabled, both on Windows and on MacOS. Developer tools are a resource drain, but bearable - the browser simply gets slower after a couple of hours of use. And then much slower, like unbearably slow, even the developer edition where I don't have uBlock or any script/flash/adblocker installed.