Do some software maintenance (more later) and check for the two know Mac specific Trojans with the free DNSChanger Removal Tool. Now the software maintenance cleaning can be done with either Onyx or Yasu and run all the cleaning routines. Just a word of advice, after running any "cleaning" routines and the software reboots the Mac, immediately manually reboot the Mac again to fully rebuild the startup/shutdown cache.
Also run /Applications/Utilities/Activity Monitor to see if any program is having a memory leak and easting up processes.
With a little luck and minus any hardware failure you can speed up your browsing experience, Good Luck.
That is why one would use the /Applications/Utilities/Access monitor to see if any program is eating up processor cycles. Plus one can see the User startup files by looking at the System Preferences->Accounts, Login Items tab.