Out of curiosity, I connected my laptop with an ethernet cable to the router and fired up Wireshark to understand and 'visualize' what's going on.
Some packets caught my attention.
I was having some TCP retransmissions from time to time to and from WAN traffic, I don't have meaningful LAN traffic btw.
I started to become worried knowing that I'm having retransmissions, but thinking it's coming from and to the internet it is to be expected from time to time... I guess? Although I find it hard to quantify if it's too much and how much is acceptable.
PS: Just to say it is not about TCP retransmissions with TCP Port numbers reused.