IFW 1/4 Speed when also Logging to disk

[BriComp]

Hi, If I carry out a backup without Log To Disk selected it completes in 1 1/2 Hrs, If I use the default Log To Disk the backup time goes up to 6+hrs.
Is this normal, surely can't be acceptable.

EDIT:
By Log To Disk, I mean Log Results To Disk.

[TeraByte Support]

nothing is written until the end so it's something else (maybe antivirus scanning .tbi file or defrag or some other disk intensive operation).