I guess the next thing to check would it be beneficial to degrade the color capacity to 8bit colors for further performance.
In general, loading an 8 bit BMP is slower than loading a 24 bit BMP. But, it may not be much of an issue anymore.
In the old days, you would never use JPG or PNGs in a game or app. Loading speed was slow because of decompression time. Now with modern processors and multicores, it may give you almost no penalty due to fast modern CPUs, but that's an entirely different thing. Compressed images should never give you better performance than a 24 bit BMP, but it can give you equal performance.
8 bit BMPs used to always be slower and something to be avoided because the palette has to be read before the image can be displayed. 24 bit BMPs don't have a palette, and load very fast. However with modern processors and operating systems, the palette handling could easily be pushed into its own thread or even another core to give a speed boost. However, 8 bit images were smaller and were widely used in gaming for that reason alone even after 24 bit hardware became mainstream. Distribution size was/is always important (which is why people will still use 24 bit BMPs over JPG or PNG as 24 bit BMPs compress smaller for distribution).
