I am having the same issues as the OP. I have not played in about 2 months and have gotten a Steam Deck during the time between as well.
I just installed NRFTW on my Steam Deck in preparation for the new update coming next week and started experiencing this issue. The issue is happening on all 4 of my existing realms I have had (I am at about 225 hours played both before and after the Breach update).
The lag issue is also happening on my PC which I found out after trying some testing from advice I got on the Discord channel. The FPS is reporting at a solid 60+ on my PC but it is seriously lagged the same as the Steam Deck.
Of note, I tried starting a new Realm and that works perfectly on both PC and Steam Deck. I would hope I don’t have to fully discard my old realms though to keep playing.