The days after the halving tomorrow and before the difficulty adjustment in about 5 days will be brutal for fees with the Runes added in. It will likely take some weeks after the difficulty adjustment for fees to settle out to a new normal.
Why?
First, the difficulty has nothing to do with the amount of spam those guys throw at the chain.
If you think that there will be a massive block slowdown that won't happen in 5 days either, because 2/3 of the period has passed, so even if we are going to see something like 20% down instantly it will only last 6 days out of which 4 are doing the weekend.
Costly, a big clog, maybe, but in no way brutal.
Plus I'm not even convinced about that 20%, we just had 4% up previously, and even right now we're 2% in the green with faster blocks, as I said in the speculation topic, my bet is on at max 10% during the first adjustment.