Based on git bisect it looks like #16028 caused a regression with interactive mode in Python 3.8, Qt5Agg backend (PyQt5=5.13.2), Linux x86_64 (at least; this is my system). The previous behavior (e.g., in 98aceb1) is that just doing:

import matplotlib.pyplot as plt
plt.ion()
fig, ax = plt.subplots()

would pop up a plot, and:

• Typing in the terminal worked fine
• Hitting ctrl-C (and then enter) had normal terminal behavior:

  >>>
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  KeyboardInterrupt
  >>> 
  

But in e6ffa5d, I get:

• Typing is very slow in the terminal (worsens with plot complexity)
• Hitting ctrl-C kills the terminal:

  >>> Traceback (most recent call last):
    File "/home/larsoner/python/matplotlib/lib/matplotlib/backends/backend_qt5.py", line 507, in _draw_idle
      self.draw()
  ...
    File "/home/larsoner/python/matplotlib/lib/matplotlib/ticker.py", line 2081, in __call__
      return self.tick_values(vmin, vmax)
    File "/home/larsoner/python/matplotlib/lib/matplotlib/ticker.py", line 2089, in tick_values
      locs = self._raw_ticks(vmin, vmax)
    File "/home/larsoner/python/matplotlib/lib/matplotlib/ticker.py", line 2045, in _raw_ticks
      istep = np.nonzero(steps >= raw_step)[0][0]
  KeyboardInterrupt
  Aborted
  

If this description is not clear I could record some screen-cast GIFs or something.