In Feynman's his introduction to the second law of thermodynamics, he said,
We know that if we do work against friction, say, the work lost to us is equal to the heat produced. If we do work in a room at temperature $T$, and we do the work slowly enough, the room temperature does not change much, and we have converted work into heat at a given temperature.
I suppose that if the temperature doesn't change, then the work done must go into the potential energy part of the internal energy of the gas in the room, but then why must we do the work slowly?
The last statement is also quite confusing. As I understand, work and heat are essentially the same thing: a mechanism of transferring energy from one system to another. So is it just an abuse of language when he says, "we have converted work into heat"?