I am using a VPN service that I have tested with my main tower pc to show connection speeds way above 150 MBit/s with openVPN, PPTP, IPsec, etc.
With my OnePlus One Smartphone, Android OS 6.0.1, Cyanogen OS 13.1.2 I get around I get around 20-40 MBit/s on my normal WIfI (no VPN), using IPsec around 20 MBit/s in both direction (I have a 450 download, 20 upload connection). But as soon as I connect through any kind of OpenVPN protocoll, my download speed collapses to 0.5-2 MBit/s. The upload speed strangely does not change and stays as 20 MBit/s. The problem is 100% reproducable with my smartphone, as soon as I activate the connection, the download speed collapses, as soon as I disconnect, it instantly ramps up again. To remind you: the problem ONLY occurs with any kind of openVPN connection, when I am using PPTP, IPsec, L2TP, etc. there is no significant decline in connection speed. And it only seems to slow down DOWNLOAD speed, not UPLOAD.