"Why did you do it so slowly?"
Translation:Proč jste to dělal tak pomalu?
It is really strange to focus so much on the participle by placing it final.
The point is that it was slow. The point is not that it was specifically dělat and not some other specific verb.
Proč jsi to nedělal pomalu? Ale já to pomalu DĚLAL!
Why didn't you do it slowly? But I DID it slowly!
However, there is little point to stress the verb so much in this question. The point is almost surely that it was slow, not that it was specifically "doing".