Hi, I also noticed this behaviour today and took a look at the source code (in composer mode and in the preview). The problem becomes apparent immediately:
When the newsletter is loaded in the composer/editor, an empty DIV line with the class ‘tnpc-row-actions’ is inserted between all TABLEs of the blocks. This doubles after each save. Earlier today, the last newsletter (that the people I support were working on) had over 2,800 of these lines… waiting time: 20 seconds.
I just looked at the changes in WP-plugin-TRAC and there were some changes affecting JQuery seek+deletions of ‘tnpc-row-actions’ and ‘tnpc-row-action’ elements. I suspect that errors snuck into version 9.1.3?
My dirty quick fix for newsletters that need to be finished right away: remove the nonsensical <div class=\”tnpc-row-actions ui-sortable-handle\”><\/div> lines using, for example, the browser inspector, then save – the newsletter is fast and clean again. Unfortunately, the problem builds up again after the next save iterations …
As a temporary solution, could we perhaps revert to 9.1.12 for now?