I've used DVDInfo and Nero DVD Speed for read test. System is NEC 3500A (2.18 fw). No read issue all the way to 16X with Made in Japan Maxell 8X (02) and Fuji 8X (Yuden000 T02) discs. All graphs are smooth at 4X, 8X, 12X, and 16X. Some were burned with Recode, others with CloneDVD2.
Note that I will encounter read issues, which could result in error with older rental DL DVDs.
If you burn a QUALITY DVD-5 disc to full capacity on the NEC and have adequate hard drive transfer speed/CPU load during the burn session, then you should be able to achieve a good transfer graph all the way to 16X with the same drive. USe 4X write to compensate for the slow HD transfer/CPU speed of older system.
For DL pressed DVDs, the drive can only read up to 7.3X. Therefore, a speed drop-off is usually related to dirty and/or scratched media. Your scan of the DL DVD media looks fine.
Burn and read using the same drive.
The CPU usage at 4X should be under 40%.