Weird problem domain vs workgroup

Hello,

I have a weird problem accessing files on my servers.

Since a few week, when a user open a file on my servers (tried on all
servers) I notice a lot of I/O on the Performance monitor, it slows up the
network and all other users are impacted. This test is done with a domain
computer.

When I try with a computer setup in Workgroup mode, it works like a charm,
no slowness at all.

I checked the GPOs but didn't find anything that seemed bad...

Tried to disable a-v but same problem.

Anyone has an idea?
Thanks