Cd reading ratings are not entirely accurate. The 48 speed quoted is the theoretical maximum which is on possible at the very last part of the disc where the disc has a larger diametre and more data passed under the head in a given revolution. Most drives start slower and build up as the drive head moves towards the outside of the cd.
Also some drives read cdr media or media that it has trouble reading at a slower speed to increase accuracy.
You didn't test the drive using an audio cd as a lot of drives also read audio cd's at a slower speed depending on the digital audio extraction rate of the drive.
Some drive testing programs quote an average time for a disc which will never reach the theoretical maximum as it takes into account the slower beginnings of the disc..
If your drive has an average read speed of 20x and you have a drive which can read a full cd in about 3 minutes. Still a good speed, I wouldn't worry about it as this is a respectable speed.
I have seen some 40-52 speed drives that were crap because they spent more time re-reading parts of the disk, they sounded like a vaccum cleaner and vibrated the case something rotten. Drives reached their limit when they got to about 40x, I have yet to see a drive beat my 40x teac for speed, accuracy and silence.