[Mainnet][Synchronization] Slow at block 7.000.000

Hi,
I started to sync with mainnet, at first, sync was very fast, but from block 7.000.000, the sync process is very slow now. I try to add more peers as well as increase disk I/O but can not overcome.
Do you have any suggestion to speed up.

> ./start.sh --delete-all-blocks --genesis-json genesis.json