7.0: Excessive lag when loading Classic Editor

Heya,

I’ve been debugging the excessive lag that 7.0 has introduced for us when using the Classic Editor. Here’s a thread with some more context:
https://core.trac.wordpress.org/ticket/64740

Details: On a few older sites, we still use the Classic Editor by means of a simple filter:
add_filter(‘use_block_editor_for_post’, ‘return_false’);

When testing out 7.0-betas, I noticed a huge lag when using the Classic Editor. It could take almost 10 seconds for the editor to be focusable (most of the page loads quickly, but it takes forever for tinymce to fully load and become available).

I’ve confirmed that disabling client side media processing makes the lag go away.
add_filter( ‘wp_client_side_media_processing_enabled’, ‘__return_false’, 999999 );

I wanted to mention that, when Query Monitor is active AND 7.0’s client side media processing is enabled, this lag is tremendous!
I’ve been timing until window.load fires (which is when tinymce becomes focusable)

With a core theme and no plugins (other than the ones to enforce classic editor and do the timing):
QM off, Handling Off: 2246
QM off, Handling On: 2816
QM on, Handling Off: 3062
QM on, Handling On: 7490 (!)

In our typical setup with some other plugins:
QM off, Handling Off: 2542
QM off, Handling On: 3751
QM on, Handling Off: 3057
QM on, Handling On: 13733 (!!!)

Of course, the primary issue here seems to be client-side media processing. But the problem becomes tremendously larger when QM is active.

Happy to answer any additional questions, perform additional tests, or provide additional debugging information. Cheers!