Ubunta is a good OS, but if you partition your drive with Ubunta alongside 8, yes it will probably take longer to boot up and slow your hard drive down slightly, unless you have a solid state drive.
Personally I don't find windows 8 all that bad, just annoying. If you are looking for mere speed you could try a small
Linux OS (like damnfastlinux) which will boot quickly, but leave out features that are quite useful (such as .flv codec)