What kind of system are you running. I know that on a UNIX system you can do this through the command line. I think it's called split, but I'm not sure off the top of my head. It does it really fast though.

UPDATE: Went out and looked. It is split that does it on UNIX, with a caveat. It parses out using what ever filename you give it with the extensions being (filename)aa through (filename)zz. If you don't provide a filename it uses x as the filename so you end up with xaa through xzz, for a grand total of 676 files max. You can, however, split those 676 files into smaller files using the same methodry. The default file length that split gives is 1000 lines, so you might have to do some math and adjust accordingly to your file size.