For JPEG you want to add fREE_IMAGE_SAVE_FLAGS |= FREE_IMAGE_SAVE_FLAGS.JPEG_PROGRESSIVE | FREE_IMAGE_SAVE_FLAGS.JPEG_SUBSAMPLING_444; and default to quality 95 instead of 100.
This results in higher quality files at nearly half the size.

For PNG you want to use PNG_Z_DEFAULT_COMPRESSION (or level 4 if possible) instead of PNG_Z_BEST_COMPRESSION.
This results in 10-40x faster saving depending on resolution.

WebP changes are in #21.
This results in 5-10x faster saving for lossless and higher quality files at half the size for lossy.