I'm going to take an educated guess on this one. MD+MCD emulation is much more intensive than MD emulation, since the MCD contains a separate 12.5MHz 68000 CPU, as well as some additional hardware that can be used to preprocess certain graphical effects into MD tile data, for use by the main VDP. Plus, there is a certain degree is synchronization that's needed (more so for some games than others), which slows down emulation even more. I'm not aware that the PS2 can decode MP3 in hardware, so I would guess it's software support. I would guess that the half speed is most likely with vsync enabled, and the emulation is not quite half as slow overall emulating MD+MCD than with just MD, which would mean that every other frame would be missed by the video renderer. I've noticed similar behavior with FCEUX when trying to run FDS games, even though the FDS doesn't have a separate CPU, it's possible that the disk emulation slows it down enough on the PS2 to cause slowdown.
Originally Posted by theelf
Chip-free/MC-free homebrew <3
FMCB 1.93 on MC for emergencies, NA
250GB HDD/FHDB 1.93/OPL 0.9.2 (GSM and non-GSM)/PS2 games, NO HDDOSD
80GB HDD/FHDB 1.93/POPStarter rev12/OPL644/GSM0.37/PS1 games, NO HDDOSD
SCPH-75001 (sitting idle, slims suck):
FMCB 1.92 on MC