After upgrading armbian to 26.2.1 with kernel 6.18.10 performances on WAN interface dropped to 20Mbps for a single TCP connection. The interface is end0, renamed from eth1, driver from ethtool -i is "st_gmac".
I used armbian-config to rollback to kernel 6.12.58 and performances are ok now.

 

Will it be fixed in the future kernels ?

 

I have just cleaned-up / changed (again) things w.r.t. network names for my Nanopi-R6C. Same as Nanopi-R4S, it has 'wan' and 'lan', where wan is on-chip ethernet and lan is external PCIe connected. Double check for the R4S, but I am almost sure. So a mainline kernel in principle just numbers them eth<x> depending on timing and other things.

New kernel behavior is that the on-chip gets named end0 and for my Nanopi-R6C the external gets named enP3p49s0, as that represents how it is HW connected via PCI-e. That is 7.0.<x-flavor> generic distro kernel. Then Armbian has 2 udev rules to rename to wan? and lan?. That is where the problem was and is for me as I use various different bootloader/firmware. I removed those udev rules as I run standard main distros as well and I need the names for bridges and vlans etc future proof and I don't care about names wan and lan (not using it as router). But if you want to keep that naming, check bootloader version (U-Boot I guess for you) and also udev rules and maybe other patches for RK3399 and/or R4S.