NYsBadBoy, you needn't change computers (You don't change the car if the tyre keeps puncturing each day). Accessing websites causing your computer to slow down can be caused by multiple factors. First you have to see if your system doesn't have any basic hardware or software problems. Windows must have been installed properly, with all configuration done correctly. See if you have an anti-virus software (updated) installed on your system. Strongly suggest that a firewall sotware also be installed and configured for your needs (of internet access). Accessing web-sites allows 'cookies' to be accumulated in your hard disc's temp folders (for certain reasons), and alot in number of them could cause the system to slow down. If you have alot of data on a large partition of hard disc, there could be disc fragmentatation, causing your system to slow down. Windows may have become corrupted as well. Possibly more, virus(es) or more (spy ware, etc)may have infected your computer. Fixing these problems should provide significant enhancement in performance. You mentioned accessing "X"- rated sites... and it could be that if you're in the U.S., the N.S.A., and / or others may be 'infiltrating' your computer or monitoring traffic from/to your system (and even like that they may be) to see the 'pattern of usage' due to you know what, which causes a slowdown in internet and system opeation. You may want to also check the setting on your browser's internet access control profiles.
If all this doesn't work, you'll have to give me the details of your system type, configurations, devices attaches/used, the windows version or else, connection modes, etc. Then I can Zero-in into the problem. The above is just a "Basic" diagnosis. All the best, Take care...!!