As of just now, I've pushed a change to add FreeType2 wrappers to yt, to enable easier image annotation, particularly from volume rendering. The function is included in yt.mods, and it's called annotate_image. (It is documented -- "help(annotate_image)".) This should be the start of much faster image writing.
There may be issues with compilation on your next update, when you execute:
yt instinfo -u
although I have attempted to guess the paths correctly, and all new installations using the install script will include FreeType2. If you run into any issues with this change, please email me directly, off-list.