i'm just running it again (a slightly modified version built with my consolidated version of the elliptic curve and chainhash libraries the consolidated versions taken from btcsuite/btcd/btcec and decred/dcrd/dcrec - i did it to get rid of the decred link!) and very pleased to see that at last it seems that checkpoints are working properly.
it's powering through, already at 100k blocks in under 3 minutes...
weeeiilllll... ok, now it's back to slow mode after that. no idea why. i know it's validating every block when its clearance rate drops under 2000 blocks/s.
gonna add the AVX2 SHA256 implementation to my modifications now, see if it helps push the block validation rate a bit more.
