Hey! The IPython console enters eventloops in a fairly different way from IPykernel, but I've done a small dive into how the IPython way works, and could have a direction to look in? Flying home for the holidays tomorrow, so it may be a little while before I can look into this more than the stuff below.

For reference (and to organise my thoughts):

The %gui magic is implemented at

ipython/IPython/core/magics/basic.py

Line 518 in a265ad1

return self.shell.enable_gui(arg)

It just calls enable_gui on the current shell.
For the IPython terminal, the shell is TerminalInteractiveShell and it implements enable_gui by setting its _inputook attribute:

ipython/IPython/core/magics/basic.py

Line 487 in a265ad1

def gui(self, parameter_s=''):
This inputhook is then used in a prompt_toolkit way I don't entirely understand to run the gui eventloop. I believe prompt_toolkit tries to run the inputhook whenever the console is idle basically. The Qt inputhook is here:

ipython/IPython/terminal/pt_inputhooks/qt.py

Line 23 in a265ad1

def inputhook(context):

More details on this inputhook implementation: https://github.com/ipython/ipython/blob/a265ad1286acbb8d8c03146eb065defe5a42d312/docs/source/config/eventloops.rst

My testing on Windows shows that pressing down a key to input a repeating character populates the terminal just a little bit slower when the qt loop is enabled.

I think the problem is the separate implementation of the loop on Windows, which uses a 50ms timer to exit instead of hooking into a file descriptor notifier like on Linux. Qt timers are only guaranteed to last at least roughly the specified time, so in reality I'm seeing inputhook times of 55-200ms. This could result in perceptible lag, as I'm definitely seeing the hook being called while the user types in the console.

The reason for the timers is explained here:

ipython/IPython/terminal/pt_inputhooks/qt.py

Lines 60 to 62 in a265ad1

    
           if sys.platform == 'win32': 
        
               # The QSocketNotifier method doesn't appear to work on Windows. 
        
               # Use polling instead.

They were added in this pull request: #9706

IPykernel happily uses a QSocketNotifier approach to exit the Qt loop on Windows:
https://github.com/ipython/ipykernel/blob/main/ipykernel/eventloops.py#L99-L106

But a quick test on my Windows machine seems to not work with the notifier approach (which should in theory be much smoother).

@AmerM137 would you be able to test if the issue occurs in a Linux/unix system, perhaps Windows Subsystem for Linux?

Next steps for me would be figuring out why the notifier thing works in IPykernel but not IPython - perhaps its a socket vs file descriptor thing? It could be that this is a very different issue altogether though - @AmerM137 is 50-200ms lag per character consistent with what you see, or is it more?

	if sys.platform == 'win32':
	# The QSocketNotifier method doesn't appear to work on Windows.
	# Use polling instead.

Uh oh!

Enabling %matplotlib qt in IPython shell slows it down significantly #14581

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