Ok cool youre up and running.
Do you have the usb program cable or does it wiork with your pie?
Its more like 52v*20 amps for a average constant of 1000 watts. Honestly you should be able to get more than 40kph even with a 15a draw.
Not many bikes need all that current to keep get and keep a speed up I go faster than you at 48v with 15a max draw,
So I'd do a run of the mill check for the usual things that slow your bike down. Like tire pressure and brakes centred and stuff.
Then you want the promissed amps. Check the program of the controller if you can. Then maybe do a high load test on that battery at 25 amps to see if there is a weak spot in the cells or
BMS.
Check connections and stuff.
Yiou do appear like you know what to do but just in case I will post this anyway.
Good luck.