Lag between applier and dest database

[Yuto Umeda]

SourceDB: MySQL 5.7.22 on RDS
DestDB: MongoDB Atlas
Extractor: 6.1.13 build 45 on EC2 (standalone, AMI)
Applier: 6.1.13 build 45 on EC2 (standalone, AMI)

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Hi,

Applier has several minutes lag when replicating to the dest database in peak hours (peak means many transactions on src database).
The reason why I thought the applier has lag is that TIME of THL log in the applier differs the timestamp of the document replicated on atlas.

For example,
TIME in THL log is 2022-04-05 09:29:57.0 but the document's ObjectId is 624c0cb205108d60ef852921 means 2022-04-05T09:32:34.000Z.

Could you tell me what load can cause the lag if you have the similar case?
I checked general metrics such as CPU, Memory and Disk about the applier and atlas, but I could not find heavy load.

Thanks 

P.S.

I'm sorry for using support portal incorrectly the other day.