A note about the config file; keysyms directly from SDL_keysym.h don't apply, as they are in decimal, VBA expects them in hex.
Converting can be done in google ("10 in hex") or with bc ("echo "obase=16; 10" | bc")
Also, with Xorg you can use the graphics card for the display (XV) with one of the YUV options. Doesn't lower CPU usage per se, but gives more cycles to actual emulating instead of drawing.