I've just done some sampling on a number of Domino 8.5.1 servers that have not be defragmented since they were build (2+ years) and it appear that 98% file fragmentation is rather common.
Further proof that when left to its own, NTFS and Domino can make a rather nasty mess.
So if you have never done a Defrag, startup the Windows defragmentation tool, point it to your Domino data volume and run an analyse. Let me know what you find.
If it is a nasty mess, give Defrag.NSF a try.