According to Breakingmuscle.com, The difference in which muscle fibers you build comes not from your training speed, but your training intensity. If you do an isometric exercise that involves holding a half-full cup of water out at arm’s length for four hours a day, you’ll develop lots of slow-twitch muscle because the intensity over time is low.
If, instead, you tried to do a shoulder raise on that same arm at maximal intensity for a fraction of the time, like seven to twelve seconds, you would fatigue the fast-twitch muscle fibers (and even all the muscle fibers) much more quickly. Intensity is key to your speed training, not the speed of the movement itself.
In this case, we’d say that for some, body-weight could be enough to train the fast twitch muscles but for someone like you who seems to regularly workout, adding weight to your exercise is a great idea. Hope this helps, Anton.