We have a WordPress 5.8.6 installation that suddenly became very slow about a week ago. Time to first byte is very high, close to six seconds. We use WordPress primarily headless, and the problem also occurs when JSON is delivered instead of HTML. However, when I open a large file (something like /wp-content//foo.jpg) directly, it loads quickly.

We installed the Query Monitor plug-in, but the database queries for a single page do not take longer than 0.02 seconds in total.

My next step would be to disable individual plug-ins at a time. Can you recommend a less invasive approach to isolate the problem?