Speed optimization, also known as link downshift, enables fallback to 100M operation after multiple consecutive failed attempts at Gigabit link establishment. Such a case can occur if cabling with only four wires (two twisted pairs) are connected instead of the standard cabling with eight wires (four twisted pairs).

The number of failed link attempts before falling back to 100M operation is configurable. By default, four failed link attempts are required before falling back to 100M.

In enhanced mode, fallback to 100M can occur after one failed link attempt if energy is not detected on the C and D channels. Speed optimization also supports fallback to 10M if link establishment fails in Gigabit and in 100M mode.

Speed optimization can be enabled through register configuration.