Well if we have it the way you are describing Mars, i believe you should have to hit a certain HP threshold before the speed reduction actually takes hold. Therefore like Unno said you can try to escape on initial attack, but if you decide to stay the whole battle then you'll be injured aka slowed. I.E. After your HP hits 75% you'll start to be slowed, but it should also have an end threshold such as once your HP hits 15% it shouldn't continue to slow. Because at a certain point you would be slowed to the point of no chance of a comeback.
So for a full example:
(not real number suggestions, just used for the example)
76-100% HP, no slow
16-75% HP, have a slow proportional to the amount of HP lost to portray sustaining injuries.
15% HP, Slow will level out so that you don't become a sitting duck just because your HP is low. (When i say level out, just for clarity, it'll be what you were being slowed by at 16%, the slow will just no longer increase.)
Just an idea to try to compromise both sides.
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