Switches and STP
Some switches use Spanning-Tree Protocol (STP) to negotiate a loop-free path through a root bridge. While scanning, it can make each port wait up to 50 seconds before data is allowed to be sent on the port. This delay in turn can cause problems with some applications/protocols such as PXE, DHCP and DNS, of which MAAS makes extensive use.
To alleviate this problem, you should enable Portfast for Cisco switches or its equivalent on other vendor equipment, which enables the ports to come up almost immediately.