TTFB very high (10+ seconds)

Dear Support,

My site has suddenly became slower, sometimes it would take more than 10 seconds to load a page. I’ve found out that the TTFB was 10+ seconds too in these cases. I’ve searched on google and followed the advices there. Trying out the plugins 1 by 1, the problem returned when I’ve enabled WP mail smtp plugin. The problem only occurs on the admin menu pages (occuring randomly, not always), is more than 10 seconds long. The occurence is totally random, it even occurs in almost empty pages (simple select query from an almost empty simple database table). It also occurs when trying to load the wp-admin login screen (before even getting logged in).
The high load happens around every 10-15 page loads or so, not on every refresh or navigation. It happens seemingly randomly, there is no real pattern. I have seen that there was a very recent update for the plugin and the problem is also present for a few days only. The website also changed IP address (because of the hosting company doing so) 4 days ago. Could you please help me how to resolve this issue? Should I roll back to the previous version, or is it not related?

Best Regards,

Peter

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