Hi David

If there are no error codes best to check the battery and then any mechanical issues that might cause it to slow. You might want to try change the throttle assembly off one to the other and eliminate that. Settings can be accessed and speed can be altered through the display with a canbus key connected to the system. It may be that previously the settings have been left so that it can be adjusted without the canbus.
If you want to add an email in your profile myself and others can forward you some information which will show you how to access the parameters settings etc.