I have checked your project and have extracted the logic of this function (almost 1:1 copy paste). It works for small databases (few GB in size). I can calculate a fragmentation percentage easily (by checking consecutive blocks of same globals). But for bigger databases (TB in size) it does not work as it only enumerate a small percentage of all globals. It seems the global catalog is quite big and split on multiple blocks (usually it's at block #3).

EDIT : there is a "Link Block" pointer to follow :

Thanks. I remember seeing it before, while looking for info about database internals (it was a long series of community pages you wrote). I will try to make that BlocksExplorer work and will post results.
 

The database is on a SSD/NVMe drive. The impact of random access vs sequential on SSD is less than HDD but it's not neglectable. Run a CrystalDiskMark benchmark on any SSD and you will find out that the random access is slower than sequential one. 

This image summarize it well : 

 

Why I want to defragment the database: I found out that the length of the I/O write queue on the database drive goes quite high (up to 35). The drives holding the journals and WIJ have much lower maximum write queue length (it never get higher than 2) while the amount of data being written is the same (the peaks are about 400MB/s). The difference is database is random access while WIJ and journals are pretty much sequential.