In the initial post, you described, that oneof the roombas starts "slow" and after a while running, it speeds up.
So, I don't think, this is a problem with dust in the optical bumper sensors.
I discovered a problem, that the "transmitter" part (Ir-LED) in the optical bumper sensor housing gets weaker within lifetime. Now the problem is, that the transmitter does not have enough "signal", to pull the line to a level which signals "bumper not pressed". When the battery voltage decreases, the sensor ("receiver") is "strong enough to pull down the signal line an it works without problem.
If you are not shure, or don't believe me: Just enter the service mode and advance to bumper check. you will see, that ,when you have a fully charged battery ONE of the bumper sensors believes to be "pressed" permanently. When battery is at lower charge state (say: below 50 or 30 percent), the bumper test displays the correct current bumper state.
After replacing both parts (Ir-transmitter AND receiver) in both bumper sensor boxes, my roomba works, like a new one. Price: approximately 50 ct each.