Add some information about lazyloading to Performance section

[tulipdexter]

Would be good to mention lazyloading in the performance section.

Might need to link off to a more technical lazyload section within JavaScript which references use of IntersectionObserver.

[dotcode]

mention lazyloading

That is … an horrifically concise way to describe an awful lot of work.

[tulipdexter]

Not as concise as what's currently in there 😆.
I don't think lazyloading is that much to add. We encourage lazyloading in <...> circumstances because <...> reasons and we implement using <...>.