Can you install a plugin like this to see if any specific plugin is causing this ?

https://www.ads-software.com/plugins/p3-profiler/

Next we’d want to check what’s slowing your database down. You’d want a plugin like this to identify any queries that take a longer than usual time to run.

https://www.ads-software.com/plugins/query-monitor/

Turn off the culprits as identified above and your site should hopefully be back to normal.