I'm not at all confident I'm reading the source correctly, or understand much about how modern rendering engines work, but:
It appears OpenXcom decompresses all sprites at startup then renders the unpacked versions. If so, there's a good chance it could be sped up a fair bit by only storing the packed versions in RAM, then decompressing them directly to the screen each time they are to be rendered.
This is because the packed sprites are stored using RLE, meaning that rendering them this way has the engine entirely skipping most transparent pixels in one leap instead of checking each one to see if it should be rendered or not. I was able to get a significant speed increase in my battlescape editor when I gave up pre-unpacking the sprites, which makes sense when you consider that well over 50% of most of them consists of empty space.