witness node synchronisation extremely slow, and never finish.

I am configuring another witness node in addition to my current one. Got it configured with the exactly same setting on the cloud instance with the exactly same hardware spec to my current one, and started the synchronisation.

Now it's been 2 days, and still around the block number 20,700,000, and never seems to be getting done soon. It got extremely slower from around the 20,000,000th block and now it seems to sync 20 blocks per 5 seconds.

Anyone has the same problem? Could it be due to the seed nodes? Which seed nodes did you use recently?

Please help.