Loading Page Running Adobe Flash Player is Slow

It's hard to say.  There are a finite amount of resources (Memory, CPU time) on the system.  Each open tab consumes resources.  Windows 8 and higher does a better job of mitigating that by stopping each tab that isn't actually visible and saving off the state, but that definitely isn't happening on Windows 7.

What it sounds like to me based on the description that the behavior degrades precipitously over time, is that the system is exhausting usable RAM, and paging memory to disk.  Disk-based virtual memory is on the order of 100,000 times slower than physical RAM.  We also require contiguous blocks of available memory, so it's also possible that you have some free memory available, but it's fragmented in a way that we can't find sufficiently large contiguous blocks, which would also land you in a similar outcome with regard to virtual memory.

Also, upgrading and downgrading operating systems is always a little scary.  It could be that something is wonky with the system state. There aren't enough specifics here to go check the behavior you're seeing against a known-good machine, but my sense is that this is specific to your system, and/or could be alleviated by only running a small number of tabs simultaneously.  If the problem persists after a reboot with only 1-2 tabs in play, then my thesis here is totally wrong, and some details would be helpful.