Sync data very slowly when count exceeds 280 millions

[Songyue-ke]

Monstache config:
version: 6.7.17
all the config in .toml are default except the mongo url and elasticsearch url config
the machine specification： 4G 4U

ES specification
1.4 TB storage 32GB RAM 5vCPU

MongoDB:
1500GB storage 256GB RAM
the count of the collection that want to sync to ES is 365millions

When the data was first synchronized, it was very fast, 17 million data is synchronized every hour. However, with more and more data import, the speed is getting slower and slower, and when the data exceeds 280 million, it is only tens of thousands of synchronization per hour.

[Songyue-ke]

Is there any performance enhancement solution?