This is a rough issue because there is no solution for it.
Although I can recommend you some things that may help with your performance:
1.) Using browser to play flash.
Browser's flash performance compared to standalone is better for some so I recommend you to try to find a browser that still supports flash.
Some examples include:
Maxthon Nitro 2015,
Chromium, (Needs to be setup to use flash
https://www.youtube.com/watch?v=YqoyTkgPQ_0)
Liebao (I don't completely trust this one as it is chinese and it put stuff on my pc, but if you want to go for it)
2.) Turning display quality to low.
This is something you can do in the game's settings. Just go to the main menu and set it to low.
3.) Setting task manager up for flash.
To do this, you would need to open task manager, set affinity to a single CPU core and set the priority to something higher than normal. This isn't guaranteed, but it's worth a try.
4.) Turning off background apps.
If you are playing PB2 I recommend you turn off anything else that could eat up your system's resources.
5.) Play in windowed mode.
PB2 does lose performance if you play in full-screen or maximized mode, so I recommend you keep it windowed and to NOT make it maximized, keep it at 800x400 as it is intended.
6.) Rivatuner.
Rivatuner is an FPS counter and it also helps to stabilise the FPS on your game. It downloads along with MSI afterburner, so you could also download that. It automatically attaches itself to the game, but I think you might have to put in more effort if you are playing with browser PB2.