Graphics aren't the issue (usually). The most likely bottleneck is your
clock speed, especially for geometrically-complex maps or slaughtermaps with lots of actors. Modern games have multithreading and rely most on the GPU to push a lot of polygons, but GZDoom relies on
single-threaded CPU performance to process the map structure and actor scripting (which is far more complex in GZDoom than classic Doom). Unfortunately there's not really any way to get around this without breaking literally everything "Doom" about the engine and starting from scratch.
A secondary problem
might be your GPU if it doesn't support more modern rendering technology that GZDoom has been starting to use, but I kind of doubt that's the case.
The only real recommendation is to play maps with fewer monsters, and/or maps with less complex geometry. The other option is to get a new CPU with better base clock speed.
