The slits have two functions.
In high rpm they hardly influence the compression ratio, but they very much do so for starting.
They also cause exhaust gas blowdown into the crankcase. If about 10% spent exhaust gas is mixed in with fresh charge, combustion will be a lot cleaner and burning will be with blue flame. This very much reduces carbon and carbon monoxide pollution.
You could also make the slit on top of the exhaust port. In that case it provides easier starting and less exhaust noise.