Dust and dirt are the biggest problem. Case fans and power supply fans pull air in from the front of the case (and some air through the drives) and blow it out the back. A lot of dust gets pulled into the drive openings, and some of it can cause read problems with the LED read head.
Take a can of compressed air, such as used for blowing dust off of camera lenses. Carefully insert the tube of the air can into the front of the drive and move the nozzle side to side while blowing out the dust.
There are also some cleaning disks that are commercially available for cleaning the drive lens. Chose and use such disks carefully, some can scratch the lens.
Dirty, scratched disks can also slow down the disk read. The drive has to retry the read several times to get all the data through the gunk. There are disk cleaning kits that can help. There is even a kit that will repolish the read side of the disk. (It is worth noting that the label side of the disk is the most fragile part of a CD - if you scratch or otherwise damage that side, the disk is pretty much permanently gone. Be sure that, if you try to clean or polish a disk, you only work with the shiny side opposite the label side.)
Finally, sometimes the read LED just begins to dim or the drive electronics begins to fail. When the head has to re-read several times, you see it as a slowdown of the drive. There is nothing to do in this case except look for a new drive - replacements are cheaper than repair.