I never bothered to figure out an exact relationship between attack rate and the speed stat, since any particular fight is also going to be influenced by your other stats, as well as the stats of your opponent.
The next time you level up, save the game before distributing your stat points, then play for a bit so you can fight a series or group of creatures. After that, reload, put some or all of the points into speed, and repeat. With just a couple of points you might not notice a huge difference (except at the start of the game), but it should make the fights a little easier.