How old is your Mac? Flash has hardware acceleration since 2010 or so.
And if you do have a Mac built in the last 4 years, you're not missing much by using Flash. Difference in CPU usage is negligible and I haven't used an in-browser HTML5 player that wasn't shitty compared to its Flash implementation.
A few months. I was about to tell you I haven't installed it due to benchmarks that show the great power drain - then I noticed that all coincide with the time period you mention. Its funny, I only have issues with YouTube, nothing else. Guess I can't justify not having it now!
And if you do have a Mac built in the last 4 years, you're not missing much by using Flash. Difference in CPU usage is negligible and I haven't used an in-browser HTML5 player that wasn't shitty compared to its Flash implementation.