But this isn't about mainframe vs Java. I have a feeling it's about Terminal user interface vs GUI.
A modern terminal app, will be just as much fast as those old mainframe apps, is my feeling.
More detail: http://bitsavers.trailing-edge.com/pdf/ibm/3270/GA23-0059-4_...
I clicked on that link hoping to see one or two pages of info that made it clear how the whole system worked with more detail than you gave, but not a whole book of minutia that I don't have enough reason to care about at this point in the discussion.
Or a video, skip forward to about 8:00 https://higherlogicdownload.s3.amazonaws.com/IMWUC/UploadedI...
Just think about the difference between an app developed specifically for the desktop (mouse) or for mobile (touch): same GUI but completely different programs and experience
Plus responsiveness. Terminal apps have faster response time than GUIs. And when someone is typing like 10 chars per seconds it matters.
The usecase is different between terminal vs GUI. Where in GUI you look - choose - decide and click. And in terminal you blindly storm on the keyboard as fast as you can.
IMO a reason is because nobody seems to bat an eye when faced with a "TUI" made out of lines, boxes and garish colors but if a GUI is made like that it is suddenly ugly so toolkit developers put a ton of effort into shiny looks.
But try a GUI made with Win32, Motif, Tk (not Ttk) or even Gtk1 and it'll be as responsive as any terminal application (of course it also depends on the application, there isn't much the GUI can do if the application abuses it or is sluggish for other reasons ).
Actually I think it is more nuanced than that.
Some GUI programs are well optimized for fast keyboard only usage even if you can use point and click. NetBeans, Eclipse, IntelliJ, Sublime Text and VS Code are on top of my mind but even some old web applications used to be quite usable I think.
