ecobike

my ebike run 55+ km/h for 80minute without any load. but with load it run only 4/5km with 20+ km/h speed. but again when i unload it run 55+ km/h speed. my battery 36volt 7.5 amp and motor 250watt. i also attached controller specifications. please can someone tell me what is the problem.

Is there anybody that could provide me with a review about the brand Ecobike (https://ecobike.com/en)? I would appreciate it more than much.