Quite disappointing really...
Somewhere after FlashPlayer v11.6 things go south.
On one of my browsers I have v11.7.252 (which I could not find for download) but is still fast,
however I tested a few versions available from the adobe archives with 11.7.260 (or some other subversions), which proved slow.
To avoid the plugin factor, I tested the download SA (stand alone) versions which showed the performance issue.
11.6 and before = fast / 11.7 and later = slow
Chipset: i3, OS: Win8, No graphic accelerator
Don't take my word for it, you can see for yourself:
https://dcprequest.herokuapp.com
Here any image corrections will re-render for about 0.5 seconds on v11.6 and earlier,
and it will take more than 5 seconds on most 11.7 and later (including 11.9 and 12.0).
(hint: after 2 sec. of rendering a message with rendering percentage tracking will appear)
... any "solutions" to this problem, other than informing the page-visitors to roll-back their FlashPlayers to v11.6 if they want fast rendering?