If you don’t know what JIT is, it’s a software component which takes application code, and dynamically analyses and improves the runtime performance of such application.
Users have previously stated noticeable performance improvements, although not uniformly across all applications.
XDA forum member scuccia, has put together some zip files that allow you to enable or disable JIT on ROMS that do not run HTC Sense. Moreover, the dev has recommended that they be applied in ClockworkMod Recovery.
For a list of ROMS on which this has been confirmed to be working and not working, and to find out how to check if JIT is enabled or not, visit the modification thread._________