Earlier quoted context omitted.
Yeah, they were like "Flash is too slow" then the replacement was 5x slower
No one argued Flash was too slow, they argued (correctly) that Flash was closed source, proprietary, and had a lot of security issues
I also remember people citing performance as a reason YouTube switched from Flash to HTML5. Searching those blogs now is giving a lot of 404s. Like I said this should've helped since it's video, but somehow YouTube immediately got slower anyway back then. Back then I installed an extension to force it to use QuickTime Player for that reason.
The proprietary and insecure parts were real problems too. I'm fine with the decisions that were made, but this was a drawback.