Is it a T5 spawner?
Drop the print commands, first off. See if that helps.
Try making the sleep command a bit shorter, but you're pretty much near the most useful value for that now.
After that, design your grinder so that the rear of the turtle is a chest, then fill the turtle up with stone blocks and remove the for loop for voiding stuff from its inventory (ask if you want me to post a modified version of the code). When it attacks something, it sucks up the loot, and then automatically drops it behind itself. Right into that chest.
That is the fastest a single turtle can be, and that works perfectly well on my T5 wither skeleton spawner.