Hi Luc,
So I guess you use monOpenDatabases and attribute AppendLogRequests, AppendLogWaits to see this ?
Do you also look on the waits for this spid with monProcessWaits or overall waits monSysWaits.
I would think the start point should be here to have log cache for log pages, and check the size of user log cache size and then how the IO performance is in the operating system for the logs devices.
Niclas