Translation:Speak more slowly, I do not understand.
Yes, according to http://masterrussian.com/verbs/govorit_pogovorit.htm
I tried and it was not accepted. I expect that they are not interchangeable. In the phrase "speaking to a doctor" the emphasis is on getting information from the expert, whereas in the phrase "speaking more slowly" the emphasis is more on the speed of the action of speaking.