I've had similar problems with the 2.5GB NIC, even with no load. Pinging from my PC to the NAS shows gaps during which the NIC is unreachable. Even this morning, with the very latest armbian image (Armbian_20.11.6_Helios64_buster_current_5.9.14.img), right after the first boot, nothing installed yet, I couldn't even get "apt update" to complete. Four times in a row, apt complained it couldn't download some lists (never twice the same) or couldn't locate a server (my Pi-Hole logs show every DNS request was fulfilled). I switched back tp the 1GB NIC and everything works like a charm. It was the same with the two previous images. Ping reports from my PC consistently show between 8% and 20% dropped packets when using the 2.5GB NIC.