No, not sarcasm at all. I tried Flash on a variety of different operating systems and browsers at the time -- and IE5 on Mac OS 9 was the only one that consistently delivered good results.
It seems weird, but when you consider the context at the time, it makes sense. Back in the late 90s -- when Flash was on the rise and being rapidly developed -- IE5 was the browser to have on Mac just as it was on Windows. The reasons had to do with the infusion of cash from Microsoft into Apple to keep the latter company afloat while Jobs restructured it and pivoted it to Mac OS X. At the time, companies such as Adobe and Macromedia -- which hadn't been acquired by Adobe yet -- developed and tested on Mac first and most extensively, because they catered to the design crowd. As for browsers -- Safari wasn't around yet, Netscape was boomer tech by that time, Mozilla was broken, and Firefox wouldn't come out for a couple years. That really left only IE and boutique browsers like CyberDog and iCab. So the environment that got the most thorough testing was -- once again -- IE on Mac OS 9.
When Mac OS X finally did come out, it would take a number of years for Mac developers to fully embrace it. Remember, to the Mac teams inside Adobe and Macromedia, Inside Macintosh was the fucking Bible. Classic Mac OS was The Right Thing; Mac OS X was something weird and wrong and inherently anti-Mac, as parts of it came from Unix, which was known for having user-hostility baked into its design philosophy. It was also the future, but it was a future they'd take years to reckon with. So, for a while, the native OS X version of Flash chugged no matter which browser you used it in. Meanwhile, IE5 for Mac OS 9 still worked perfectly well provided you launched the Classic environment, so if you wanted top Flash performance it was still available.