Desktop client slow scan

I have pretty big sync folder (sometimes more than 150 Gb) with huge amount of files (1.5 million), and several Windows clients. Update portions sometimes include changes for 20 - 30 thousands of files, and Desktop client (nextcloudcmd also) scan them very slow, about 3-7 files a second. disc load is like 0.1 Mb/sec, but one whole processor (3770k) core loaded (and this is going for 5-6 hours). Is nextcloud (nextcloudcmd/csync2) ready for this by design? I learned about rclone some time ago, and has amazed by speed sync in async. But rclone does only dumb “robocopy like” synchronization, not updating local database, so it usable only in “push to nextcloud, then sync back” regime, which is not so good and require me to have two copies of data. How can i improve native sync speed? Can i launch several nextcloudcmd’s with one database using in multi-access mode?