When a sector is found to be bad or unstable by the firmware of a disk controller, the disk controller remaps the logical sector to a different physical sector. In the normal operation of a hard drive, the detection and remapping of bad sectors should take place in a manner transparent to the rest of the system and in advance before data is lost.
There are two types of remapping by disk hardware; P-LIST (Mapping during factory production tests) and G-LIST (Mapping during consumer usage by disk microcode).
There are a variety of utilities that can read the SMART information to tell how many sectors have been reallocated, and how many spare sectors the drive may still have. Because reads and writes from G-list sectors are automatically redirected (remapped) to spare sectors it slows down drive access even if data in drive is defragmented. If the G-list is filling up, it is time to replace the drive.
In other words, if your hard drive has enough remapped sectors, it will be slowed down, even though the hard drive continues to work.
If your hard drive has SMART capability, check to see how many sectors have been remapped. If the hard drive has to get part of a file from a different part of the hard drive, it will slow things down. The more sectors have been remapped, the slower the drive. Defragging with the windows defrag tool will not fix a slowdown due to remapped sectors, though there may be some 3rd party utilities that do.
If there is no slowdown EXCEPT when using IE on the Internet, then the problem is IE. But check out other web browsers to see if they run faster or if they have the same problem. If they have the same slowdown as IE, then you know the problem isn't IE.
Another possibility is if you have an antivirus that is checking everything on the webpages you visit before allowing IE to load them. If that's the case, the speed of your computer will matter because your antivirus will be using processing power for scanning.