Speed (and Jump) are introduced in level 15. The speed markers are noted by a set of 3 colored arrows and you will speed up if you are the same color when you touch them.
You will easily get both cumulative achievements pertaining to speed just by playing the game but if you're looking to grind it out, level 21 has 2 spots where it can be done very quickly. One is directly after the first time you change planes. Just don't change color or shoot the blocks and your ball will bounce from the block to the speed marker continuously. There's another spot soon after where drag blocks are in your way and you can bounce from the speed arrow to the drag block and back again.