Really? Your original question was:
Again: Enable SpeedRead.
If the drive still doesn't speed up then it has problems with the quality of the disc that prevents it from going full speed.
No. It will allow faster reading. No drive function can magically make a disc better. Whether full speed can be reached will be dependent on the quality of the disc.
IMHO that is good news. At least it looks like your drive is not defective but the disc is crap.
Paper labels are a very bad idea. It's impossible to apply the properly centered and the stickers weight itself is usually not regularly distributed over it's size, thus leading to an unbalace that cases vibrations that prevent high speed reading. Even more paper stickers can expand or contract leading to a deformation of the disc (lifting up the outer rim) which can lead to the exact problems you are describing.
Different drives act differently. Some die when they see fingerprints, some do harakiri when seen scratches. Apparently Plextors don't like warped discs.
Seriously: Neither. Many people get to forums like this one and report problems with the 'if it happens to me it must be an universal truth' attitude. Now the question is, does that make people that don't happen to have the same problem persons that care so much about their integrity that they life in denial (or, in this case, Plextor fanboys)?
I (or others) have no problems like you describe (but then we don't have the very same disc to try). That only tells us that the problems you have are not necessarily universal but your own. Your own in regard to both drive and disc in question. You simply may have wound up with a combination that doesn't work. That does happen now and then, also with other drives (and other discs).