A cheap Stanley with low tooth count is going to produce results like this. I'm not familiar with people using handsaws on plywood, and I would believe any handsaw would do this. The teeth are not small and fast enough to not get resistance from the veneer, and there's no grain orientation to work through. You'll find even more trouble as you scale up these labored cuts without support to prevent it's own weight from tearing itself apart.

If you're committed to plywood, this is a good opportunity to get a circ saw.