Hi JJKK,
Firstly, you might consider watching the following two videos, which specifically address the topic of single strokes:
Developing Finger Control:
Single Stroke Technique
Secondly, you might also consider a book of technical studies that utilise the techniques you are learning, as opposed to simply repeating them on a bad.
This book includes many single-stroke studies that apply the technique in a variety of ways, specifically designed to engage the fingers (as the videos above show), and to improve control, which itself includes speed. The Long Form studies in that book ensure consistent repetition of the motions for every subdivision between 8th notes and 9-tuplets.
Thirdly, in my experience, you are not repeating for long enough. I would suggest slowing the tempo slightly, and aiming for a minimum of one minute per run. You might consider the following exercise:
1. Choose a 10 bpm tempo range. This might be something lower like 120-130, or something faster like 170-180. The upper number needs to be about 90% of your max. If your goal is 200bpm and you are nearly there, you might start with 170-180, for example.
2. Set the metronome to the lower number, so 170 in this example, and play the singles at a consistent volume as 16th notes for one minute.
3. If you
successfully complete the minute, increase the tempo by 2 bpm (172), and go for another minute. If you
fail to complete the minute, reduce the tempo by 1bpm (169). So +2 for a success, -1 for a fail. Your goal is to get to the upper tempo within a single practice session. In theory, this can be achieved by performing five successful one-minute runs in a row. However, if you are working at the upper range of your control, you will likely go up and down a few times during the session.
Your own definition of success and failure will dictate whether you count each run as successful or not. You might just about scrape through the minute, or you might just feel the control wasn't quite there, or you may find the whole minute easy. But the overall goal is to encourage
focus during the repetitions. You need to be actively engaged and providing yourself with analytical feedback over longer periods. 10 or 20 seconds simply isn't long enough for the feedback loop to take place.
Ideally, you would do the above exercise for larger periods - perhaps 2.5 minutes or 100 bars of repetition. In all cases, you need to be focused, providing yourself feedback, and correcting the errors through evaluation of technique.
Speed will not result from simple sprint drills. You have to assess the technical weakness, fix it, improve the control, and build it up gradually.
Good luck!
Jonathan
EDIT: Sorry, I reread your post and might have misunderstood. It sounds like you are doing longer periods of repetition, not just 10-20 seconds at a time.