In theory you're right. In real life, it's unlikely that the developers who were (probably barely) able to implement the system in a high-level language are able to rewrite and it in assembly code. And even if they were, unless you don't intend to change the code ever again, it's more likely you're just piling up technical debt.

Assembly isn't a good example, there are plenty of high level languages which produce excellent compiled output. But you have to use those, not any fancy new language or framework.

But your point is correct in that in many projects there isn't enough budget to make the product faster. Which is a project management decisions, not that the speed issues are unfixable.

Are you seriously suggesting that there's no improvement to be had in the typical ui without rewriting in assembly?