Once you changed distro all bets are off. There is no way I can image you could easily narrow this down to ext2 vs. ext4. Distro devs make all sorts of decisions about what to/not include in the final package. Including such things as desktop options, I/O scheduler, (background) file indexing ... - as well as mount options.
Have you converted that ext2 to ext4 (on Zenwalk) to compare the effect ?.