My optical drive has suddenly slowed

[R.I.P.]

Symptom:
My optical drive has suddenly slowed to a crawl reading discs, and it refuses to read some discs.

Solution:
Optical drives usually don't expire gradually; most simply stop working without so much as a death rattle. It's much more likely your drive's lens has accumulated a layer of dust.
Get yourself a can of compressed air at the local geek emporium, and open the disc tray. Spray into the drive with quick, short bursts at an angle (so the dust is more likely to be expelled from the drive). Do not spray continuously or with the can upside down, because that could introduce moisture into the drive. Give the dust a minute to settle, and spray the innards again.