Are you on a laptop? It may be Optimus that applies this side effect.
Maybe there is a sort of 'energy saver' that lowers clock.
I'm still on win7, but in win10 there may be an energy option where you can tell to not save energy on a 'performance' profile, which i set up everytime I installed a desktop with win7 to max. performances
I remember an old tool: rivatuner, which allowed to change the default clock speed of GPUs, but i dunno if it still works nowadays.