I had the same issue when I first got a legal ebike. You can average 11-15mph on the flat without breaking a sweat. Up a decent hill then you'll have to put in 50-60% of the work you would on a standard bike. I also found that the electric assist on its own was too slow, I got impatient and would end up putting in 95% of the work, hence arriving to work sweaty.
Had I let the motor do all of the work, I would be far slower than someone on a pushbike, but it can be done, just with some extra effort on the hills.
This is why for years I have always said that it would be far easier and attractive to have the laws changed to 500w motor, but still restricted to 15mph before cut-out, as the only times you really need the power is up hills, and the legal power limit at the moment is only beneficial on the flat, where you don't really need it at all