I expected TFTP to be slow on IOS. Switched to SCP and I'm topping out at ~45Kbps. I get consistently slow speed on 3560(X), 3750(X) and similar vintage devices.
sh proc cpu hist suggests the switch's CPU is bottlenecking on the encryption cycles. I looked at changing the ciphersuite to a less cpu-intensive one such as RC4, but didn't get very far.