both of your dvd rom and dvd burner must be in DMA mode to achieve the maximum speed, also this applys to dvdshrink, decryptor, as dvdcopyplatinum only reads and burns at there speed, not your decision. hope this helps
The 106 (in common with most writers) is firmware locked to a maximum ripping speed of 2x. You can use hacked firmware to rip at maxumium speed.
However it sounds like your devices are all running in PIO mode anyway, so you need to sort that out first.
It will rip faster towards the end of the disc (hence the increase you saw)
Hacked 106 will rip at the same speed as the ROM drive.
But better to use the ROM as it saves wear and tear on the writer.