Many things can effect computer "speed". Eliminate "startup" or Login programs: from System Preferences->Accounts:Log in Items.
Often programs install themselves here without the user knowing it. If one of these login programs is waiting for an internet connection, and there is none, it could slow your machine. If you have corrupt files or fonts, it could slow your machine.
If your processor is starting to fail, it could slow your machine. If the thermal paste is drying out (due to age a possibility) it could slow your machine.
You could be having a slowdown due to overheating with the larger HD in it.
If this Answer is helpful please remember to return and mark it Accepted.