As of Lumberyard1.10 we have been getting huge slowdowns/stalls when working with the material editor. The problem turns out to be caused by some new code in CMatMan when you have lots of CGF files which do not have default materials.
This function CMatMan::OnFileChanged is called whenever a material is edited. In that function there is a loop which calls Refresh(FRO_GEOMETRY); on any loaded statobj which is using the ‘Default’ material. For us this has been causing stalls of anywhere from 5seconds to almost a full minute whenever you change any material setting.
I have a workaround for us but wanted to point it out in case the issue affects anyone else.