Could you confirm whether there are length tags inside the files? I guess many ID3 taggers do display the length fields somewhere (TagsRevisited does it in the main view, ID3-TagIt in the tag window, Details tab, for example).
With scan_type set to 2, the scanning doesn’t take much longer though; a few minutes (less than 5 for 20k+ songs on a slow NAS) the first time, that’s all… so maybe you should switch to 2 and tell it through the admin interface to do a full rescan.