If we are always adding enough $s anyway, why do we need a more complex prefix?

Just to reduce the possibility of duplication.

Instead of trying until we add enough $s, can we instead do this and only compute it once?

let $count = 0;
const re = /\$+(?=\d)/g;
let m;
while (m = re.exec(tpl)) $count = Math.max($count, m[0].length);

const prefix = "$".repeat($count + 1);

I guess includes() would be a bit faster? Of course it doesn't matter.

The difference is that with includes we iterate over the string n times (where "n" is the final number of times we have to expand the prefix), while with the regexp loop only once (because the regexp alwags start after the previous match, and matches prefixes of any length).

It seems that only in the last case (very long code and $$$$$$0) the regex is slightly faster, while in other cases includes() is faster.
It should be faster to use BABEL_TPL$ as per the current code.
perf