- As I noted about the directly-connected PC
Regarding the stock firmware:
Many have observed that phenomenon as well and noted it in the forums. I speculate many reasons for it, it is mostly attributable to software improvements in OSes (i.e. the Linux Kernel) tend to add some some CPU lag. I observe this increase even as I upgrade versions of the same OS on PCs. One major known cause of latency in OpenWRT as compared to stock firmware is noted in-depth here: Hardware NAT For LEDE