Why does it accept "my speed is small", but for "viteza mare" it does not accept "big speed"?
I think it depends a little on the context. I've heard/used both low and slow and conversely both high and fast.
"Slow speed", "low speed" and "the speed is slow"... Almost never "the speed is low"
In Spanish we use to say "alta velocidad" (high speed, more formal or literary), "mucha velocidad" (a lot of speed, more colloquial) or "rápido" (fast).
"Velocidad rápida" (fast speed), instead, sounds a bit redundant and thence is not a preferred form.
I keep my speed down to avoid accidents: I drive at low speed, not at high speed. When I leave the traffic lights I speed up and when I approach a sharp bend I slow down so English is full of idiosyncrasies but "My speed is low" (or small) is just wrong in English, unless you are speaking about your stock of a certain illicit drug, called speed...