A GFS filesystem is split up in several resource groups (RGs). The number of resource groups depends on how big the filesystem is, you can check the number by doing a
gfs_tool df <mountpoint>
What probably is causing the slowdown is that somebody is trying to create files. The sequence of lock operations to do this is:
Acquire an exclusive lock on the directory.
Acquire a shared lock on the resource index
Wander through the resource groups to find a single RG[...