Benefit of simply linking or benefit of setting to null and then setting the recordsource after the client's record is loaded?
subforms load before the main form. So if you load a large recordset into a subform for subsequent filtering it will take longer than loading a small recordset. You are also transiting much more data across the network.
When you say "change of record", that means setting the recordset using code rather than letting Access handle it by linking?
whichever - when you change the record in the main form.
Shadow, these are not fixed rules. You design and tune the app on the basis of what it is going to do. Basic principle is a recordset (be it one created in vba, a form, report, combo or listbox recordset) accessing data from anywhere will load quicker with a small amount or zero data compared with a large amount of data.
An app where the user opens a single form and spends the entire day on that form browsing through the records may not benefit from opening it with a zero length recordset - just take the 'initial hit' and it's done. But that same form which is used by users who frequently open the form to a specific record selected from a navigation or other form, do what they need to do and then close it, will benefit in the initial form opening. How much of a benefit depends on the below.
How long a recordset takes to populate is also dependant on how well the underlying tables and relationships for that recordset are designed - this includes consideration for normalisation and indexing, not using domain functions in your queries, incorrect use or design of outer joins, union queries and group by queries and perhaps other factors as well.
But usually the biggest factor is the speed of the connection between the front end and the backend. A local backend is clearly going to be the fastest, one on the network will be slower because it is dependant on the speed of the network and standard of your connection points. Connecting via VPN will be slower still because you have the performance of that connection as well.
So anything you can do to reduce the amount of time spent transiting data across the network will show a benefit (and also free up the network for other users). How much of a benefit will depend on the above factors with biggest benefits being seen on large datasets over a slow network.
Many newbies are not aware of these factors. They start developing in the fastest environment (backend on their machine) and it is only when they go live and put the backend on the network that these performance issues become evident. a not uncommon reaction is to blame access and decide they need to move the backend to sql server. But if it is a straight transfer, little benefit will be seen because the real reasons for slow performance have not been addressed.