Trying to play Dungeon Hack, and I'm having problems with the keymapper.

Using the arrow keys which are the default controls in the game, everything runs fine. But when I map the controls to WSAD through the DosBox keymapper the controls are slow and unresponsive.
I have to hold W in for a second or two to get a response, and then the game will jump several steps ahead straight away. Turning is also affected, hold the turn key a few seconds, nothing happens, then when it "hits" it spins 180-360 degrees at once instead of just 90 as it should. (Dungeon Master/EoB style game).

So I tried on my other PC, with the same problem. Then I tried a few different games and had the same results. Keymapped controls are slow, unresponsive and VERY annoying. 😒
I've tried deleting the original keymapping from the remapped keys, so they all just have one mapping, but it makes no difference.

Any ideas?