BTC Fullnode: Why do the last 10% take so much time to sync?

I'm setting up an Umbrel Node. It's syncing since 5 Days now and at ~94%. But those last 10% really take a lot of time. Is there any explanation as to why that is the case? The blocks do not contain severely more transactions than 2017/2018 blocks did (both were full), but it takes significantly more time now.

Is that because the longer the blockchain becomes, the slower the validation process will take, the further the progress? Or am I syncing with a lot of nodes that aren't complete themselves yet (cannot really imagine that this is the case)? Or is every single block validated against the whole tail back to the genesis block?

Or what's the reason for that? Thanks you all that help here! :)