I have a strange situation : I updated a website to Wordpress 5.9 and suddenly the page loading time increased from 1-2 seconds to 15+ seconds. Looking over the network tab i see that the TTFB is increased to 11+ seconds.
I created a test domain and installed WordPress 5.8.3. Added the same theme with no content - just the "hello world" post. The loading speed is 0.6 seconds . If i update to 5.9 the loading time becomes 11+seconds.
Here are 2 screen captures from query monitor
5.8 -0.6 seconds
5.9 -11.6 sec
The server details are
Version 7.4.27
SAPI fpm-fcgi
max_execution_time 600
memory_limit 512M
upload_max_filesize 128M
post_max_size 128M
Server Version 5.5.5-10.3.32-MariaDB-0ubuntu0.20.04.1
Extension mysqli
Client Version 70427 (7.4.27)
I start disabling portions of code and i think i can tie the big loading times to get_header and wp_head() functions but i could not pinpoint the problem. I also disabled all the hooks and filters but with no luck.
I checked the database queries and the load is not coming from there - you can see from screen captures also.
I disabled Gutenberg styles on front end
I read the release log for 5.9 but i did not saw something that could affect the website.
Any advice will be appreciated since i'm running out of ideas.