I tried this patch on 2 devices. On my MT7621 I am seeing LAN-WAN speeds around 930Mbps as expected with the sirq maximum at 47%. But if I do a reverse iPerf (only difference adding -R) my WAN - LAN speed is only 250. Without fast path LAN - WAN around 350 so definitely a good improvement to use the patch and WAN-LAN the same around 250. My ISP only gives me 200/20 so no problem, but seems strange to me.
On an MT7628 with only 100M ports, speeds are more or less the same with or without, but SIRQ is about 15% lower with fastpath. Does it still make sense to use it (to reduce system resources used)?