Low Upload Speeds on the NanoPI R2S

First of all, I have pretty novice knowledge of networking in general so please, feel free to correct me on any misinterpretation/misconception I may have, just be patient in doing so.

As stated in the title, I'm experiencing low upload speeds. Something in the range of 20Mb/s, with a peak at 60Mb/s sometimes.

It's a PPPoE 500Mb/s down - 250Mb/s up fiber connection.

I can hit, and most times surpass, these speeds when using my TP-Link EX220 (Stock Firmware) as a router or plugging my modem directly into my desktop and setting Windows to dial the PPPoE connection. So I know my ISP is indeed delivering the speeds.

I'm aware that the NanoPI R2S is not the most powerful device to run as an OpenWRT router and I shouldn't expect much higher transfer rates using it. But since it can hit 480Mb/s download speeds on average, with peaks at 510Mb/s. The 20Mb/s upload range seems way too low, which makes me wonder if I'm missing something. Speed testing was done by plugging my PC directly into the LAN port of the router.

I already enabled software offloading and Packet Steering which indeed seemed to help, since upload speeds wouldn't go higher than 10Mb/s before it. Hardware offloading is enabled as well. Irqbalance is installed too. Is there anything else that can be done?

I couldn't test it with iperf because I couldn't find a comprehensive guide teaching how to do it. If someone has one, it'll be greatly appreciated.

To test with iperf, you can find a PC to pretend as WAN internet, then connect your R2S WAN to it, LAN side with another PC, both with iperf3 installed then you can test.

I own R2S and I'm sure there is no issue on R2S to handle ~600Mbps upload/download speed.