You will not get anywhere near 100 MBit/s while running a VPN client on your router, see Comparative Throughput Testing Including NAT, SQM, WireGuard, and OpenVPN for a rough idea (ath79 should be roughly comparable, if you account for its ~50% higher clockspeed and multiply the benchmark results by ~0.65 (no, you can't compare it quite that easily, but it will give you a ballpark figure - at least for VPN performance)).
VPN throughput maps pretty closely to the pure hardware performance, as neither ramips/rt3883 nor ath79 have any hardware acceleration for it. This also shouldn't differ that much between different firmwares (OEM (assuming it had VPN support) and OpenWrt or other alternatives). So your router's hardware performance will be the bottleneck for this purpose.
Do read https://openwrt.org/toh/asus/rt-n56u before flashing anything, as not all hardware revisions of this particular device seem to be supported.