You don't get speed with muscle; strength is not equal to speed. Mozart, Liszt, Chopin, etc., were farthest from he-men. A sumo wrestler is slower than most midgets. A fast Samurai is not built like a sumo wrester. If you want speed, practice for speed, not muscle.
Exercises are the worst things for speed. Of course, a beginning amateur will need to practice SOMETHING in order to get some speed, but you are not a beginner any more, so you shouldn't practice like one, and fingers can move only so fast and you can't physically accelerate them to the speeds needed to play fast -- IT IS A PHYSICAL IMPOSSIBILITY. So why even try? Exercises, especially FF on heavy pianos will only get you muscle bound with too much flexor muscles and not enough extensors. So you have already broken 3 of the most important rules for speed: (1) heavy piano action, (2) exercises, and (3) no etudes. Those are the 3 worst things you could ever have done, and so you naturally are suffering the consequences -- no surprise at all!
If you want speed, get a lighter piano, even a digital; but, of course, nothing beats a properly regulated grand. Then learn the use of parallel sets (link below section II.11). Don't play FF until you can play fast. Then learn to accelerate hand, arm, motions so that you can combine them with small, slower finger motions in order to produce speed. Most people concentrate on finger speed and completely neglect the fact that the hands and arms are much slower than they can be. Needless to say, learn to play relaxed (another reason for using light piano and no FF). Needless to say, all speed work should be HS until speed is reached of exceeded. If you do these things, you will at least GET STARTED in the right direction and have a chance at success. There is, of course, a lot more.