In my experience my 2009 MBP didn't slow down as the battery wore down. But I did read somewhere that if you remove the dead battery without replacing it, then the MBP will only run at about half speed on mains power alone. Depleted batteries have a habit of bulging and damaging the internals. I ended up buying a third party replacement battery, but it never restored the MBP to its original run-time capacities.