lacp rate fast command is used to change the rate at which LACP control packets are sent on the LACP link. By default, this value is set to 30 seconds, but this command will change that to once every second.
Typically, you shouldn’t need to change this setting, and it should under normal circumstances remain at the default value. However, if you are in an unstable network environment where you expect these physical links to flap occasionally, it may be useful to allow LACP to “reconverge” quickly.
If you are in a stable network environment, it is recommended that this command be removed completely. The port-channel will function correctly, however, other features such as high availability and SSO may not operate correctly. Also, this feature should not be used on vPC peer links.
More information about this command and its implications can be found here:
I hope this has been helpful!