I'm not so sure about this. When the JVM first came out, it was slow as hell; by your logic, this was an indication of a design flaw which would plague the JVM

I'm not so sure about this. When the JVM first came out, it was slow as hell; by your logic, this was an indication of a design flaw which would plague the JVM forever. Well, the JVM is blazing fast now. They worked around most inefficiencies. Facebook was built on PHP and MySQL (remarkably slow technologies), yet they scaled this to one of the largest systems which ever existed. If an UI is slow, surely you can find a more efficient way to draw the elements on the screen, or to reduce their resource consumption, or just give the UI better hardware. You can always optimize. Some design decisions make this hard, but I think you can always work around them.