Since I own several GL-iNET routers on hand and just upgrade my home line to 1G bandwidth, so I did a series of performance tests under 1Gb(dl) / 600Mb(ul) PPPoE line, the results are for your reference.
Test environment and description:
1Gb/600Mb PPPoE fiber line
tested devices act as the direct connected to the ISP MODEM main router
use Ookla Speedtest-cli as performance measuring tool run inside devices (choose the ISP testing point)
test a couple times to take the best performance result
If VPN profile is active, does the router not have to process the traffic to determine if the traffic goes down the tunnel or not? So even if it is not directed down the tunnel, the processing introduced might limit speed based on the CPU? If you try it without VPN profiling, might the speed go up?
I think some of these chips use offloading to reach high speeds, and that gets disabled when something like this is introduced into the picture.
Ookla Speedtest is mainly intended to measure broadband Internet speeds, which can have noticeable variations. For measuring router throughputs (without VPN), I use self-hosted LibreSpeed or OpenSpeedTest on the LAN only.
For comparison and reference only, I made 3 runs each on GL-A1300 and GL-MT2500A (my GL-MV100W is in already use).