I want to mention a couple important things about using soft start features with your miter saw. First, is a safety precaution. Miter saws without a soft start feature reach full RPM very quickly and you don’t really have to allow the blade to speed up, because it happens usually in under a second. This speed is why it’s tripping the breaker in the first place.
With a soft start, you must allow the miter saw to reach its full speed before making the cut. If you don’t you can cause your miter saw to bind, this can damage the material, the miter saw blade and the internals of your saw. Give it at least 3-5 seconds to reach full speed.
The next point is that soft start adapters aren’t cheap. They start at $80 and can easily reach $200 for home use soft start adapters. Not cheap for an addon feature I know. However, if you’re having this problem with your miter saw, you’ll likely see the same problem in other applications in your home.