Consult the owner's manual that came with your log splitter. This will give you the full specifications of the splitter, including engine size, force and torque.
Locate the "force adjustment" dial. If there is not such an adjustment, to change the speed would require changing the engine power which is not recommended and highly dangerous.
Turn off the splitter before making any adjustment. You do not want to turn any dials while the splitter is running
Adjust the dial to a lower force or speed. Depending on the model you can do this one step at a time until you reach the desired speed. It is better to start low and work your way up.
Start the motor for the splitter and test the new speed with a log. Repeat the steps until you reach the desired effect.