NoelCan,

my blind guess would be that your computer is low on physical memory. CPU load doesn't bring a modern OS to its knees, high I/O load and especially memory paging however can severely impact performance, especially when switching between applications. After quitting applications that used a lot of memory, it will also take the system a while to get back on track with paging the other applications back from disk to physical memory, which would explain why your system is slow for a while even after quitting Poser. Looking at the Activity Monitor screen shots you posted, my guess would be that your computer has 1GB RAM? In that case, you should consider adding more RAM to your computer, it will make not only Poser but the entire system feel more responsive. I upgraded both my Macs from 2GB to 4GB a while ago and it made a significant difference.

Another thing to check for is if you're rendering large textures without filtering. Since Poser then cannot use the texture cache nearly as efficiently as with filtered textures, it will cause significantly higher I/O load.

Snow Leopard may worsen the situation because since most applications that come with it run in 64bit, there is the extra overhead of keeping all system frameworks in memory twice (32bit and 64bit instance) as well as programs running in 64bit using a bit more memory for larger address pointers.