I did some timing on the exact speed of jumps using this map I made: https://www.robotwantskitty.com/75254. They seem to save 4 frames each if there isn't a block directly above your head. If there is, it doesn't save any time and might be slightly slower, although you can get the 4-frame boost forward if you jump into the corner of the blocks. You land about 5 blocks away from where you started your jump if you don't have any block above your head, 4 blocks if there is a block 3 blocks above the ground, and 3 if there is a block 2 blocks above the ground. The second jump in a double jump don't seem to give you any boost.