Both were absurdly slow and resource intensive.

Nothing got my old iMac G5 whipped into a frenzy like loading a page that had Flash on it somewhere. I’m not kidding when I say that there were many areas in World of Warcraft, a full-fat seamless open world 3D MMORPG, that didn’t make its fans as angry as Flash could manage to.

I was so happy when it finally started to disappear. That kind of sheer disregard for my system resources is inexcusable.

I wonder what could have been if it had had the same effort applied to optimize it as JavaScript did.

It probably could’ve been great, but unfortunately such a thing would’ve been unlikely at best with Flash being solely controlled by Adobe. Not only does that company have a disinclination towards optimization that persists to this day, but with there having been no alternative implementations of Flash, there was no possibility of other organizations taking up that torch or for there to be competition between implementations driving things forward.

My experience circa. 2008/9 was it was fine (used it on a commercial app in lieu of good enough web standards for the featureset). It was fast and reactive for an app-like experience.

Games were made in flash so it can't be that slow.

Maybe slow for time to load. I think we got that down a bit and this wasn't to render content but a utility for a signed up user.

It was OK on Windows. It sucked on any other platform. And it was a security disaster.

Flash was a lot of things, but also, at the same time, neither of these things. (You could come across poorly coded and thus poorly performing apps, but that wasn’t the standard).

Modern day SPAs are far, far worse offenders when it comes to smothering resources.

Just like browsers now.

The flash player leaked memory. The longer-running the app, the more noticeable it was. I believe this is why Adobe Air (flash player packaged as deliverable) failed.

As for the speed of the flash player itself it was quite good.

They were resource intensive (though probably less so than yr average React based site these days) but Flash, especially post AS3, was an order of magnitude or two faster than JS + html/svg/canvas at the time. It was years after the death of Flash that standards based tech finally caught up.