if you have grease on the gears inside gearbox.....clean it off and run dry!
Make sure nothing is binding in the drive train.
Once your straight line speed is the same you need to sort out the handling......and for that you will have to read and then take what you have read and put into practise by trying different settings.
....edit....Ok I see you mentioned the handling is ok.....you may need to lookm at it when you get the speed up
