You tempt me to try submitting a patch. I suspect it's a bikeshed problem, but I could be wrong. There is no technical reason why you couldn't enable it only on elements with a "-webkit-hyphenation" CSS rule. Then it's not a regression, it's an optional feature.
You are right that there are real perf concerns. A web page reflows multiple (sometimes hundreds) of times during pageload, unlike a print document.
TeX was designed to work on computers of the late 1970's. It seems pretty reasonable to think that algorithms that were practical to execute then can be carried out more or less instantaneously on machines available today. If you think you can pull off a port, by all means try.
You are right that there are real perf concerns. A web page reflows multiple (sometimes hundreds) of times during pageload, unlike a print document.