Could it be that TCP requires a little more CPU processing than UDP, with CPU being the bottleneck in VPN speed, hence the difference in speed perception?
With that said, the MT300N-V2 with single 580MHz CPU gets ~10Mbits/sec, while the BL1300 with quad 717MHz CPU gets ~24Mbits/sec with the same setup. I wish GL-iNET will release a new BL1300 firmware for VPN to take advantage of the quad CPU. From what I see, VPN is a single core operation across most if not all routers.