And UI is a bad example IMHO. UI with a high response time which is uncomfortable (or even frustrating) to use is probably the result of thinking that UI performance doesn't matter.
My opinion is that performance as well as security is very hard to retrofit if it was not considered from the beginning. And if you have necessary knowledge it doesn't take much more effort to create software which is faster (or more efficient) and secure compare to software created by an ignorant developer.
I heard many times that one should write a program first not thinking about performance, then profile and optimize a few hotspots. But in my experience 2nd part (optimization) rarely happens and people who can profile and optimize usually also can write faster code from the beginning.
It is painful to see how people who don't know and don't want to know typical latency [1] and throughput numbers, who don't know about algorithm complexity and may other things design and implement very inefficient software.