If the battery is removed from a MacBook or MacBook Pro, the computer will automatically reduce the processor speed. This prevents the computer from shutting down if it demands more power than the A/C adaptor alone can provide.
This also applies to Macs with a non-removable battery such as the MacBook Air and Retina MacBook Pro.
My MacBook Air battery has just given up after only six months and the performance has decreased dramatically – to the point I can’t actually believe this is the same Mac I used yesterday. As an example, a python script to generate HTML files for a Pelican blog would normally take about 2-3 seconds to execute. Since the battery failed it’s a good 10 seconds before it’s finished.
As much of a pain in the ass this is, I’d rather a crippled Mac than one where it tries to use too much power and cuts out during work.