1. Garbage collection and minimum runtime
Garbage collection decreases memory efficiency, browsers need a certain minimum amount of resources to render a page, no matter how complicated leading to 500MB RAM text editors like Atom and worse once you open files. Similar problems plague Eclipse and IntelliJ which both often consume 1GB of RAM. The JVM often needs 150MB for an empty or perfectly optimized app.
2. Everything is an object with pointers
This is especially bad in Javascript where every object is basically a hashmap. This causes performance slowdowns because even something as simple as looking up a field is a pointer chase through several layers now. Raw numerical data may consume a lot of memory if you are not using typed arrays. Especially bad with ArrayList<Integer> in Java.
3. JIT
JIT compilers can only spend a certain amount of time on optimizations, which means JIT languages tend to either suffer from slow start up times or faster start up but less optimizations.
4. GUI complexity
Things like having animations and constantly recomputing layouts.
If you designed your processors for these things and made them fast at this, the only further source of slowdown is a lack of caring because you have already exhausted the unavoidable technical reasons. E.g. your processor is so fast, you write a naive algorithm that takes 5 seconds on the fastest computer available but then takes 20 seconds on an old computer.
Diffing a VDOM might fit here, but that's not really GUI-specific - just a combination of your earlier points.
What does that look like?
There is a reason Apple doesn’t do #1 and #3 and is moving away from #2 in their own code.
They are just inefficient ways of doing things. Designing a processor to support an inefficient mechanism will still lose out to a processor which doesn’t have to.
Look at multi-platform products like ms office and how slow they run on a Mac. I suspect because there is a translation layer to bridge win32 calls to Mac equivalents. And that seems like it would be point 5 on your list.