SQL Server: Using Dual Databases for Performance?
Two things


You don't actually say you have an index on the table -- I expect this would just solve your problem.  An index on f_application, f_computername, f_endtime, f_starttime should make your update time tiny with only 6mill records.
If you want to split it up, don't do it the way you describe, create a table for open but not closed records that you use before your current table.  Then when something "updates" delete it from the that staging table and insert it in your big table.  Using a staging table in this way is considered a leading/best practice - having a table arbitrarily split in two is always a nightmare to work with