After investigating into the D2(X(-XL)) menu rendering code, I have found a way to remove CPU intensive computing overhead and thus could greatly reduce menu rendering lag with the new menu style enabled.

An example:

On my Athlon 64 XP 3500+ s939, 1 GB RAM and a X800 XT PE frame rates in my level Speed! (windowed, 640x480) dropped from around 72 fps to 30 fps when calling up the options menu. With the code changes, there is a drop to between 60 and 70 fps depending on what parts of the level are in view.

The improvement should be very apparent on older machines with less RAM and a weaker CPU, but a good video card, because as I said already the problematic code burdened especially the CPU with a lot of computational overhead - unnecessarily, if I may add that.