As someone who has training in boxing (and some, albeit limited training in karate), I have to disagree based on experience. If two people are the same weight, height and strength but one of them is a professional boxer and the other has zero fighting training, the boxer is going to hit harder because the boxer knows where to strike, and how to strike.
It doesn’t mean you yourself are stronger, but your strikes will have more impact due to you knowing and having training.
And just to make sure, I checked the wiki and it appears that Speedsters do have super strength. Though this could vary depending on which form of media/which universe (Prime Earth, Arrowverse, etcetera) but Speedsters do have superhuman strength in Prime Earth.
If you’re referring solely to metas who are connected to the Speed Force, I think they do have super strength when using the Speed Force. Barry has been shown to lift very heavy objects without much strain. Otherwise, I’d say about as strong as a professional fighter of the Speedster’s height and weight.
A Speedster’s super strength isn’t comparable to others where that’s one of their main powers, though. It also may depend on how much training a Speedster has. Barry for instance, was trained by Wonder Woman, so he could very well hit harder than Wally West I, because he had no such training.
Their connection may also play a role in it. So if we were to take two Speedsters who were equal in height, weight, and fighting ability, but Speedster A has a stronger connection to the Speed Force than Speedster B, then Speedster A could be stronger.
I could very well be wrong about all of this. Or eight about it. Take it with a grain of salt.