Calculate concurrent user sessions on Access DB

Would suggest an index on the startTime_f field.
If that doesn't help, perhaps you can move over this table to SQL Server Express (free edition).
Then Access is capable of executing Pass-Through queries really fast.

First, create a query that lists the timeslots used:

SELECT 
    UserLog.StartTime, 
    UserLog.EndTime
FROM 
    UserLog
GROUP BY 
    UserLog.StartTime, 
    UserLog.EndTime;

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Save this as TimeSlots.

Then find the count of users having used any of these time slots:

SELECT 
    TimeSlots.StartTime, 
    Count(*) AS Users
FROM 
    UserLog, 
    TimeSlots
WHERE 
    (TimeSlots.StartTime Between [UserLog]![StartTime] And [UserLog]![EndTime]) 
    OR 
    (TimeSlots.EndTime Between [UserLog]![StartTime] And [UserLog]![EndTime])
GROUP BY 
    TimeSlots.StartTime;

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Save this as TimeSlotUsers.

Finally, find the maximum count of concurrent users:

SELECT 
    DateValue([StartTime]) AS [Date], 
    Max(TimeSlotUsers.Users) AS MaxConcurrentUsers
FROM 
    TimeSlotUsers
GROUP BY DateValue([StartTime]);

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Of course, indexes on the time fields is a must.

Thanks very much Gustav. This works perfectly. Only one little question though - I have user session data for multiple applications on the same table. These queries work perfectly for concurrency for one application.

When I did try to customise this for multiple applications, it comes up with the output of different applications listed, but it doesn't consider each application as unique & does not calculate concurrency for that application, but considers only the time fields and therefore provides the concurrency for the overall database.

Can you please help with any thoughts as to how to list max concurrency for each application?

Much appreciated

Thanks very much Gustav. It worked like magic. Can't thank you enough.

Thank you so much. Incredibly helpful.

You are welcome!