Ambient temperature can have an affect, but more likely it's something like dull blade, charge contacts, etc. If you let it go fully dead, even one time, it will affect run time, but just keep running it - eventually it will recover.
Cleaning oxidation off the charge base contacts needs to happen each Spring.
We have recently posted troubleshooting guides and Winter Service information here:
http://www.paradiserobotics.com/dealer-informationThere is a "Short Run Times" troubleshooting guide and others, also. Even clogged wheels will impact run time.
As for "service menu rapid charge" in the service menu, I have not heard of that setting before. I would have to investigate it and report back.
RobotLady