How do I trace objects on the heap with JS 59

Hi,

I'm currently trying to update my embedding from a very old version (1.8.5) to 59. I have lots of objects which I used to root using JS_AddObjectRoot.
These are stored on heap structures.
I'm trying to understand how to do the rooting in 59.

In the GC rooting guide on MDN at https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/GC_Rooting_Guide#GC_things_on_the_heap it says:

GC thing pointers on the heap must be wrapped in a JS::Heap<T>. The only exception to this is if they are added as roots with the JS_Add<T>Root() functions or JS::PersistentRooted class, but don't do this unless it's really necessary.  JS::Heap<T> pointers must also continue to be traced in the normal way, which is not covered here.

OK, so I need to use JS::Heap<JSObject *> and these need to be traced but how to do it isn't covered in the guide... Please can someone point me to some documentation/reference which says how to do it. I can't find any information anywhere...

Alternatively it looks like I could use the JS::PersistentRooted class to do what I want but it specifically says not to do this unless it's really necessary. Why is that then? Is it some sort of performance issue?

Many thanks

Miles