To add to Jake’s excellent reply, here’s a chart that illustrates how the output of a radiator (per square meter) varies with flow temperature 
A hotter radiator will output heat faster than a cooler one.
Taken from this excellent blog post on Using Radiators with Heat Pumps | Protons for Breakfast - recommended reading if you’re looking to upgrade your rads.
You may also find this (long) topic useful: My heat pump is too big, what should I do?
To summarise your options:
- run at a low flow temperature and cycle the compressor (2-3 times per hour is fine)
- run at a high flow temperature and cycle on the room thermostats
- fiddle with settings to convince heat pump to run at minimum output level
- install bigger radiators to keep up with heat pump and run at lower temperature