Well-known unpredictable TCP-over-TCP performance would only explain OpenVPN/TCP results. But OP has posed also OpenVPN/UDP performance that is 2x...3x slower

Well-known unpredictable TCP-over-TCP performance would only explain OpenVPN/TCP results. But OP has posed also OpenVPN/UDP performance that is 2x...3x slower than SSH port forwarding, so I'd guess there is more to this story (see also andmarios's impromtu test results above).