I am trying to understand if Implicit sharing capability can be used to avoid record locks or increasing performance of update operations on master-detail records.
I am referring to this link here. I am particularly trying to understand how the highlighted description here( which says Not used when sharing and the details of the 'Child' type of sharing) would help avoid record locks on updates of an Account with a huge amount of Contact records.
Controlled by Parent isn't the same as Implicit Sharing. Implicit Sharing is when a user gains access to a child record and also gains read-only access to the parent (e.g. a contact is assigned to a user, and thus gains read-only access to the account). Implicit sharing only has an effect when the parent object is Private and the child is not Controlled by Parent. Controlled by Parent, in contrast, grants read or read-write access to the child records when access is granted to the parent. It has an effect in all parent share modes (private, read-only, and write).
Using Controlled by Parent will decrease row lock time on the parent, but not eliminate it. There has to be a lock on the parent while the children are being updated. Controlled by Parent will increase performance because there's no child share table while in this mode (the children's access is checked using the parent's share table).
You can use this if you want to, but there are typically better ways to reduce row lock contention, such as making sure there's only a few hundred children per parent instead of lumping millions of records under a single parent, which is detrimental to performance anyways.
Implicit sharing is a system built feature and cannot be implemented by any other means (Manually or by automation). Most of these sharing permissions are applied across Account and it's related entities.
For your highlighted scenario, it is applied on parent and child account records and is not applicable to any other objects.
