The Z80 is a chip that complements the 68000 processor in the NeoGeo machine and handles primarily audio tasks. The old Z80 emulation core was written in the C programming lanugage, which has to be translated into machine code the ARM9 processor in the GP2X understands. The new core is written in Assembly (ASM) *for* the ARM9, so it runs very significantly faster than the old C one. That's what DrZ80 is.
If you don't use audio, you will see 0 acceleration. In games you DO use sound with, gameplay should be smoother. Bear in mind that lag where the game does not become choppy is NOT your GP2X slowing down and the program lagging-- it is the REAL MACHINE it is simulating lagging, which happens on the actual NeoGeo.
Also bear in mind games that perform heavily scaling and rotation will see very little benefit from a faster Z80, these operations will get a little more headroom to operate in but will for the most part remain the speed they are now. For example, Metal Slug and Blazing Star, also Puzzle Bobble, gain significant speed increases from the DrZ80 core, which is much faster than the old non-ASM core-- but games like Samurai Showdown 4 with heavy scaling do not benefit very noticably from it.
I for one find the improvement quite apparent; I can now run Metal Slug at the full 60 FPS the vast majority of the time at 299 MHz, before, I averaged in the high 40's or 50's.