I am curious if or how much better this formula performs:
DATETIME( INT((FLOAT([Date Time]) % 1 + 0.00000000001) * [Time Bin kettan]) / [Time Bin kettan] )
The constant 0.00000000001 was added to correct a rounding issue.
The values in [Time Bin kettan] are changed to:
See more about this in DATETIME() for rounding or truncating to nearest time interval
Attached Workbook Version: 9.0 (downgraded from 9.3)
That shaved off 16 sec in querying~ enough to warrant as an answer.