Usually not - it's a pain in the bum to take apart a laptop sufficiently to stick a new CPU in.
There are no methods of making a "USB CPU" either.
Some (but not all) laptops will allow you to upgrade the memory too - this is usually achieved by unscrewing a panel on the underside of the machine. Obviously you have to be careful which type of RAM your machine uses.
You may be able to upgrade the internal hard drive of the laptop - it'll be a 2.5" drive and some laptops allow access to it via another panel (old Dells used to do this, but my current Dell doesn't allow it).
If you've had the laptop for a while, then a good way of speeding it up is to reformat the hard-drive and reinstall the operating system (assuming it's a Windows derivative). That blows all the cobwebs out of the hard to get at corners :-)
You could also use it as an opportunity to optimise the services that start up at boot time too. The website below has loads of tips and tricks on that.