I just wanted to post there here in case anyone has this issue. I think this is set by default, but just in case it isn't or you accidentally disable it and don't realize it until later.

Doom originally played at 35 fps and the gzdoom source port does the same thing. You can disable this by doing the following.

1. Press ESC.
2. Go to Options.
3. Go to "Set Video Mode"
4. Enable "Rendering Interpolation"

Enjoy!