How to lock fields with vba just in header, not in footer

[ADAL]

Hello, as you know, in large documents that have variable headers [STYLEREF], Word becomes very slow, so the solution is to lock the fields, and after that open them (close to printing, etc.).
I do it with the command:

ActiveDocument.Sections(1).Headers(wdHeaderFooterPrimary).Range.Fields.Locked = False
ActiveDocument.Sections(1).Headers(wdHeaderFooterEvenPages).Range.Fields.Locked = False

The problem is that this macro also locked in footers, and I only need it in the upper headings [I know that I can add a command that will unlock the footers, but for a reason that is a bit complicated to explain, it is not relevant in my case] I am wondering how it is possible to give the command directly only to headers
Thanks