Macbook Pro 15 inch Late 2011 - Internet always slow until I change DNS lookup address
I can think of 4 possible reasons:

Bad hardware config

It's worth re-setting the NVRAM in your machine by holding Command, Shift, P and R while booting. You'll hear a chime - keep holding, then you hear another chime - let go. This is safe to do.

Bad/Broken Network (Filter) Driver

There's also a possibility you have a bad network driver installed, perhaps against your knowledge, or maybe a leftover from some software. Try booting in Safe Mode (hold Shift as your machine boots) and see how the internet performance is (everything will be slow in this mode, but if it's a driver issue you should see a difference in the browsing).

Malware Infection

The last time I saw this behaviour turned out to be a rampant malware infection on the computer which was saturating the internet connection by sending out spam email. Changing the network/IP details temporarily interrupted the malware until it work up again. You may be able to see this in Activity Monitor (all processes, look at network tab, anything using a lot of networking while you don't do any browsing?). It is worth doing an anti-malware and/or virus scan to eliminate this risk.

It's time to do a fresh install

Have you upgraded your OS from Leopard thru Mavericks? Ever done a 'nuke-and-pave' / erase-and-reinstall? Might be worth it in your case - you can restore your data from a TimeMachine backup (make sure you have one!), but do not have to restore the whole system. It depends on how much software you'd have to spend time reinstalling, but this has worked well in the past for people I've worked with.