Connecting to other macs in my office


I have 15 macs in my office... I see them. When I click on my hard drive icon, a window pops up and to the left it says "DEVICES" I see all the macs I can connect too. I double click on the coworkers mac I want to connect, I enter the username and password and Im connected. .. all seems good.

Heres the issue:
Some coworkers computers this is instant connection, other coworkers computers this can take up to 2-3 minutes to connect. Why?

All are connected through the same router/switch, same network
all ethernet.

All macs are the same 2011 27" imac, running 10.6.8