ksoftirqd taking a lot of cpu since moving to debian 12

I have a small pc linux box like intel nuc dual atom where I run some firewall settings. Previously this machine runned debian 9 and everything worked fine. I could have used the machine in middle of 2 others to send data at very high speed around 50-70MB/s transfer rate (gigabit ethernet). Now since I moved the setup to debian 12 my rate dropped to 9-10MB/s, by looking for an answer I found that ksoftirqd was taking a lot of cpu. I tried raising the MTU from 1500 to 3000 but I achieved only a small improvement like 16 MB/s (while still maxing out the cpu) and some adapters I'm using doesn't support MTU over 4000. My opinion is that there's something else that is not working correctly... 2 main things have changed from debian 9: - iptables is now nftables - the driver on that machine if I remember correctly was installed from manifacturer, now is provided directly in debian 12 via the non-free-firmware package what settings or log can I inspect to solve the problem? can be some settings be changed to allow more resources being used (like ram) in order to allow longer irq queque and improve the throughput or tell nftables to treat all subsequent packages of the same type in the same way speeding up everything up?