High query response time for find operation in mongo 4.0.27 with mmap storage engine with random intervals (5/7/12/20 hours)

Scenario - We have application which is doing CRUD operations on database. From past 2 years we are using mongo 3.6.9 and system is working fine without any issues but when we upgraded the database from 3.6.9 to 4.0.27 and then we started getting high query response time for find operation but it is not consistent in nature ,sometimes we are getting high query response in 5 hours or 7 hours or even 11 hours.