Slow/unresponsive as of late

[ktmn]

Describe the bug
Theme 2017, disabled all plugins except Gutenberg. Went to the demo page, selected a paragraph, held down "a" key for like 5 seconds, the whole thing froze for 30 seconds at least. Just typing text is also very unresponsive. Dev tools were not open at that time.

There is a warning in the console:

Warning: Unsafe lifecycle methods were found within a strict-mode tree:
    in Unknown (created by WithSelect(Component))
    in WithSelect(Component)

componentWillReceiveProps: Please update the following components to use static getDerivedStateFromProps instead: t

componentWillUpdate: Please update the following components to use componentDidUpdate instead: t

Learn more about this warning here:
https://fb.me/react-strict-mode-warnings

Don't think it's related though.

In chrome dev tools I went to Performance tab, set it to record, then held down a key in a paragraph block for some seconds. It's telling me Event (keypress), Event (textInput) and Event (input) handlers took upwards of 500ms.

Is that related to React strict mode?

To Reproduce
Steps to reproduce the behavior:

1. Type text
2. Is it responsive?

[aduth]

Performance may suffer some while running in development mode, though certainly not to the extent that you've seen. To consolidate, I'm going to close this as a duplicate of #6466. Let's continue discussion there.