|How old is the battery? |
Does it show as fully charged?
There are defaults the system may drop too when on battery power only; to conserve the power resource... Have you checked to see how the whole power system/management is set up within the OS itself?
Meanwhile this discussion appears to circle around the same situation:
Possibly it may shed some light...?
I found it via a google trawl using:
windows 7 ultimate runs very slow on laptop on battery mode
Perhaps repeat my trawl and have a browse of the assorted responses too?