ZDoom

Turning off dynamic lighting also helps a lot.

Thanks Rachael, your mod helped a fair bit.
I did some testing and map 15 is now giving me 30-50 fps.

This mod was actually designed on that very exact map.

Keep in mind that using this mod means things *are no longer vanilla* and there's no guarantee it can retain the original difficulty of the map when it is active. What it will do, however, is make distant monsters less active so that your CPU has a little bit better time keeping up with the amount of activity going on. It should boost your FPS and allow the map to be fully playable on most CPU's.

Sunder's a pretty massive system hog, especially if there's a lot of active monsters going on, and if I recall correctly, MAP15 is an especially infamous example of that. Unfortunately, what you're running into is a limitation of the Doom engine itself, since it can only fully use one CPU core (otherwise the deterministic nature of Doom's ticker code no longer applies, breaking multiplayer and demo recording).

Your machine, a third-generation i5, would probably be in about the right bracket for Sunder to be "slow but still playable." If you wanted to improve things, you'd need a CPU with stronger single-core performance. The 25% CPU usage you're seeing is likely one fully-used core out of four available.

To my knowledge, there's no existing Doom port that can fully use all available cores.

I am running into slowdowns and stuttering when playing Sunder map15, I looked at task manager and noticed GZDoom was using 25% of my CPU quite steadily and may be maxing out one core.
I took a quick peek at the other new maps and none of them seemed as bad, but I have not played them fully.

I have a i5-3350P 3.1GHz CPU and a GTX 1060 3GB Video card.
I do not have dynamic lights turned on.

Does anyone have any tips that could help my with this?