Slow traffic, with captive portal active, between green and blue networks


I’m facing an issue with traffic speed between green and in blue networks.

I had before my printer/scanner device in my green network and all was working nicely. Recently I’ve decided to move the printer to the blue network to facilitate printing by Wi-Fi devices in blue the network. But I could notice that with captive portal active the scanning from a device (e.g. using the “Simple Scan” tool on a Linux system) from the green network is very very slow (unusable). If I deactivate the captive portal on the blue network, then document scanning from the green network is working as expected.

Can somebody give me a clue on what could be the problem and eventually how to solve it?
I’m using “IPFire 2.25 (x86_64) - Core Update 147”

Thank you,