I did some testing a while back (January 2017) that may help you with some reference numbers on throughput. These are my speeds using the OpenVPN client on two different ASUS routers. Both routers running Asuswrt-Merlin 380.64 firmware. Connecting to PIA VPN servers on port 1198.
AC3100 (1.4 Ghz dual core)
CTF (Cut Through Forwarding NAT Acceleration)
DL: 61 Mbps with core 1 at 25%, core 2 at 75%
DL :74 Mbps with core 1 at 30%, core 2 at 85% with mods*
UL: 84 Mbps with core 1 at 35%, core 2 at 100%
AC68U (1.0 Ghz dual core)
CTF enabled
DL: 44 Mbps with core 1 at 30%, core 2 at 80%
UL: 58 Mbps with core 1 at 40%, core 2 at 100%
For reference, when using the same PIA VPN server with a windows client I'm able to attain 250 Mbps down and 350 Mbps up on the same DSLReports HTML5 speed test.
The speed tests were conducted over a wired connection from the computer to the router.
Data encryption: AES-128-CBC
Data authentication: SHA1
Handshake: RSA-2048
*Adding the following lines to the custom configuration bumped the DL speeds to 74 Mbps.
sndbuf 524288
rcvbuf 524288
push "sndbuf 524288"
push "rcvbuf 524288"
Update 5/5/2017:
Did some more testing with the windows client and was seeing the same 250 Mbps down but upload speeds increased to around 450 Mbps. I got those speeds from each of the CHI, Midwest, East and NYC servers.
I would think your 1900P would get speeds close to my 3100 as they both have the 1.4 Ghz cpu.