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If a designer knows some of these tips at the beginning of the design process, and is able to complete the initial design with them in mind (assuming time remains constant), then I'd argue "premature optimization" isn't an issue, but the end output is still faster.

Premature optimization still takes a hit - the cognitive effect of optimizing these things is definitely an issue.

No optimization is intrinsically premature. Most of these are not things you need to worry about, but there will be cases where it makes a different.

That said, specifying image sizes is often a good idea in order to prevent the page from moving around while it loads. It looks a lot cleaner if your text all flows correctly and then the images pop into place.

What developer isn't getting 90+ page speed scores these days? That stuff is trivial low-hanging fruit that rarely requires rearchitecting your application or page.