t2 classes are unsuitable for high work loads, by design. Once you have used up all your CPU credits, performance is abysmal. Again, by design.
Don't use t2 (or any t*) instances if you care about performance.
As far as I can tell, there is no way to monitor the CPU credits on RDS (the way you can on EC2)
rds_superuser, unlike a real superuser, doesn't inherently have permission to create roles.
You will have to have the other rds_superuser (or anyone else who already has createrole) do:
alter user new_user createrole;