[USB Install] Xbian is incredibly slow, even when overclocked to Turbo.

[postdeath]

Software
XBian version: Beta 2
XBMC version: 12.2
Overclock settings: Turbo (issue also seen using stock Xbian OC)

Hardware
Power supply rating: link
RPi model: Model B, 256mb
SD card size and make/type: Kingston 16gb Class 10 SD Card, Integral 16gb USB 3.0
Network (wireless or LAN): Wireless
Connected devices: IR receiver (for MCE Remote), keyboard, wireless adapter - all via a powered USB hub.

I installed Xbian last night (USB install), and after spending all day sorting out my TV library, I have found the UI to be incredibly sluggish (long transitions between screens, unresponsive/laggy input), using a number of different lightweight skins (including Confluence, Amber, Maximinimalism). It seems to temporarily hang every so often when navigating. My media library is currently sub 150. I was concerned that it may be overheating, but it's currently sitting at 53 degrees on Turbo overclock.

Any ideas? This is a brand new install, with the library rebuilt from scratch (rather than imported from OpenElec).

Update: there is no issue with video playback, it's incredibly smooth.