Afaik let in JavaScript also has a sizable penalty too, if not quite as bad as const. Var on and on.

Yup. Anything layout-related could be costly to read, and could force a layout to occur if modified since the last read (even within the same synchronous javascript code). Most things are not deferred until the next layout pass, which is one of the reasons virtual DOMs got popular: it batches changes for you.