Rasplex is a great project, but the UI is still too laggy for my liking...especially navigation between menus. I have tried other distributions like Openelec with Raspbmc, wich gives better results in terms eliminating lag. But I much prefer The Plex home theater interface. After trying many tweaks, I'm getting to something close to what i'm looking for.
What I do:
-Use a usb (3.0) stick to run my system. It gives a major performance boost, plus more stability (especially when you overclock).
-I overclock by editing the config.txt file, with these settings:
arm_freq=1000
core_freq=500
sdram_freq=500
over_voltage=6
...with heat sink.
The whole system is much more usable. Navigation is still a bit jerky, but for example fanart always appears immediately.
I'm also sure that when we have an update based on a newer version of Openelec, there will be more improvement.