WordPress Admin slow since update to 5.9.

[markhowellsmead]

The admin area is markedly slower than before the update from 5.8 to 5.9. This is noticeable with and without any plugins active. The problem doesn't appear to be on the same screen all the time. We have also been warned by the hosting company that the load on one of the sites has suddenly increased; also occurring at irregular times. (See screenshot as an example.)

[markhowellsmead]

Screenshot of server CPU/memory usage

[markhowellsmead]

Apache
PHP version 7.4 and 8.0.
Various custom themes
Various plugin combinations across several sites; also in a plain installation with no plugins active.

[markhowellsmead]

I'm not sure whether this is coincidental, but I've noticed that loading e.g. the edit page screen with the browser console open and the option for “disable cache” activated provokes the very slow performance. It may be that, if the browser sends no-cache headers, something is happening on the server which takes a lot of memory.

[markhowellsmead]

Site 1 (with WPML)
First request (with browser caching): 1.2 min
Second request (with browser caching): 14.78 s
Third request (with browser caching): 12.88 s
First request (deactivated browser caching): Timeout after 37 s
Second request (deactivated browser caching): 40.72 s
Third request (deactivated browser caching): Timeout after 56 s

Site 2 (without WPML)
First request (with browser caching): 1.3 min
Second request (with browser caching): 6.31 s
Third request (with browser caching): 10.90 s
First request (deactivated browser caching): 7.23 s
Second request (deactivated browser caching): 1.3 min
Third request (deactivated browser caching): Timeout after 54 s

[indysigner]

We do have a similar issue since updating to 5.9. The WordPress search on ​https://www.drweb.de/ became 3-4 times slower than before, which is a not so great experience for visitors.

Server-Info: Apache
PHP-Version: 7.4.27
PHP Post Max Size: 20 MB
PHP Time Limit: 60
PHP Max Input Vars: 1000
cURL-Version: 7.64.0, OpenSSL/1.1.1d
SUHOSIN installiert: –
MySQL-Version: 5.7.36-1

We do have 6,000+ articles and custom posts on the site. Still, the onsite search was fairly snappy *before* the most recent update.

EDIT: We just rolled back to 5.8.3. and the site search has become much faster again. So it’s definitely WP 5.9. that causes the issue.