What kind of speed can one expect? I've got gigabit fiber and really do get > 950Mbps in both directions. Would this throttle me to 200Mbps? I can't imagine I would be getting full-speed connectivity.
RPi4 is plenty fast for full gigabit VPN performance. Its ethernet interface should also easily reach 950 Mbps. Although it's a different matter whether current VPN software can take full advantage of it. My guess is not.
There's some handicap due to lack of useful crypto HW in RPi4. But if multiple cores are used, it should easily reach 1 Gbit speeds. VideoCore VI could theoretically also be used for crypto acceleration, although I haven't heard anyone doing it — yet.
Edit: Just tried "openssl speed -multi 4 aes-256-cbc" on RPi4.
aes-256 cbc 224787.70k 243743.77k 250572.29k 251253.42k 253684.39k 252919.81k
In other words, 2 Gbit/s CPU based AES-256 performance.
Even if it could do a theoretical gigabit...you'd still be sharing that up & down.
I suspect you could get a good 700ish with a USB 3 gigabit dongle though. I ran a rpi4 as router/fw that way for a couple months (250 internet so never found out where the limits are)
With Wireguard you should have a much better performance however as it's multithreaded and rpi4 offers you 4 cores.