How to choose, in one paragraph
If you do not know where to start, take a one minute words test and look at the two numbers that are not speed. Accuracy below 95% means your problem is errors, so spend a week on accuracy ladders and weak-key drills. A low consistency score with decent accuracy means you are stalling on particular keys, which is exactly what the Weakness Doctor is for. Good on both and still slower than you want? You need stamina and reach practice — longer runs, and code mode for the symbols.
Practice is not the same as testing
A test measures; a drill changes something. The quickest way to waste a month is to take a one minute test every day, watch the number move around by five WPM for reasons that have nothing to do with typing, and conclude that practice does not work.
Keep the ratio at roughly three drill sessions to one measurement. The drills page sets out what each drill is for and how to build a week from them.
Where the clever bit actually is
Two modes do more than shuffle a word list, and neither of them calls a server. Custom Topic matches what you type to a subject area and assembles a passage from sentence banks that came with the page. Weakness Doctor scores every word against your own error history and samples the worst offenders.
Both run entirely in your browser, which is why they are instant, free, and work with the network switched off. Nothing you type is sent anywhere — see the privacy policy.