> Am I understanding correctly that there’s a significant difference in performance between using a ViewComponent + a partial vs. a ViewComponent which renders html via a tag - from inside the component?

I don't think there will be much difference at all in everyday use, but some libraries that value performance don't avoid templates for that reason, Pagy for example.

https://github.com/ddnexus/pagy

Personally I omit them in my projects whenever we want to customise attributes, I hate seeing stuff like this in templates:

    <h2 class="<%= heading_classes %>" id="<%= heading_id %>">Some header</h2>

I'd much rather see:

    <%= tag.h2("Some header", class: heading_classes, id: heading_id) %>

And as lots of the components allow attributes to be customised, using the tag helpers directly seems like a good choice.

> Also - the gov.uk project makes my heart sing ;)

Thanks :) Although I just reimplement the components and forms in Rails, the proper hard work is done upstream.