Heavy write activity on SSD nukes system performance

This has been a known issue for awhile. Using an SSD-tuned FS like Btrfs might help, but it might not.

Ultimately, it is a bug in the IO scheduler/memory management systems. Recently, there have been some patches that aim to address this issue. See Fixed: The Linux Desktop Responsiveness Problem?

These patches may eventually make their way into the mainline kernel, but for now, you will probably have to compile your own kernel if you want to fix this issue.