I've tried every setting, reverted TAP9 drivers, turned off encryption, and have never gotten speeds over 100mbs. Is this a limitation of the TAP9 driver?
Yes, although the 100Mbps is only a coincidence that it tops there and depends on your CPU's performance. I've gotten up to 150 with mine on PIA, but it is overall an issue with how the driver is designed.
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More in this topic: https://www.privateinternetaccess.com/forum/discussion/28909/isp-speed-upgrade-made-no-difference-in-vpn-speeds-pia-speed-cap
Upstream bug reports: NDIS 6 and NDIS 5