For how many hours is it syncing already? It’s expected to get slower, as with time there were more blocks produced. I guess the % is based on the slot number, so in the beginning it’s fast because there was not that much data.
Btw you can configure your node to use more than 2 cores (IIRC that’s the default). Search for “RTS -N” or see discussion here: How long does it take to sync a new cardano-node server running v1.29?