Try this at your own risk - It Worked for me after a complete system reload which didn't make any difference.
I will take no responsibility for any damage or loss caused to you or your property. Do it of your own free will and at your own risk.
I did this on a MacBook Pro Early 2011 - 2.7 GHz Intel Core i17
System Version 10.6.8 Snow Leopard
Running well on mains power but dramatically slowing down when running on battery power
First try this to see if you get the same result I got
Quit all programs
With the mains connected
Open Activity Monitor and click on CPU in the bottom half of the window
Check the CPU Usage graph - It will probably be minimal - Under 1%
Now disconnect the mains and watch the graph shoot up and CPU activity around 80%
Here's what I did...
Again - I will take no responsibility for any damage or loss caused to you or your property. Do it of your own free will and at your own risk.
Firstly, you’re going to need the model identifier of your Mac,
go to the Apple in the menu bar and select "About this Mac"
When the About this Mac pane appears click on "More info…"
Select Hardware in the left panel
In the right panel make a note of the Model Identifier e.g. "MacBookPro8,1"
Now open a finder window and click on Macintosh HD
Click on system >Library > Extentions
Find the kext file called IOPlatformPluginFamily.kext
Right click it and Show Package Contents
Click on contents
Click on Resources
Click on PlugIns
Right Click on ACPI_SMC_PlatformPlugin.kext
Right click it and Show Package Contents
Click on contents
Click on Resources
Find the the plist file that corresponds to your Model Identifier e.g. "MacBookPro8,1.plist"
Move it to the trash and delete it
Restart and everything should now be fine.