I can't be 100% sure, but maybe your SD card isn't capable of having multiple operations? Sometimes the write/read speed isn't up to par. I used to have a PC that could do single-player emulation fine, but some multi-player games dropped fps to flip-book levels.
Try Dios, using USB Loader/CFG to forward, USB instead of SD, etc,.
By the way, in the time of your first post to your third post (~five hours), if no one responded, you might want to look for alternatives.