Hopefully, this can shed some light to the issue...

Although it's true that [generally] installing apps don't affect the responsiveness of the device, there are some apps that do affect the responsiveness, although I can't determine why. Sometimes, the slowness doesn't have any relation with CPU/memory usage at all...

For example (and this is based on my personal experience, btw), about two months ago, installed a lot of apps, including the Extra protocols plugin for Conversations and Contacts 0.7 (take note of the version), and everything was fine... until I upgraded it to 0.8. After rebooting the device, every transition went choppy. CPU/mem usage seems to be okay, but every UI animation I see in the device was slow (edit: okay, not slow, but you can easily see the difference in the "smoothness," or lack of it). It irritated me, so I uninstalled it. After reboot, the issue was gone.

A few days later, I decided to install Pidgin protocols plugin for Conversations and Contacts 0.8 (a subset of the extra protocols plugin), and I haven't experienced any lag. I personally concluded that there's something else in the extra protocols plugin that lags the UI, and it's surely not the Pidgin protocols.

I can't surely say that there's a problem with the extra protocols plugin (it actually does the work quite well), but at least in my device, there's something in that plugin that makes the UI lag. As to exactly what, I can't tell.