Your collection has no supporting Index for this query. The whole collection (2.1M documents) is being scanned, reading from disk where necessary(slow). Based only on this query an optimal index might look like:
{"deviceSessionID":1, "deleted":1,"startTime":1,"endTime":-1}
An index like {"deviceSessionID":1, "deleted":1} or even {"deviceSessionID":1} might be less optimal but could support more use cases.