Between the time of your message until now, Google is clearly the one who made the most requests. I don't know if it is the culprit, or just a combination of that and the amount of visitors (and also the database getting always bigger, thanks background useless cars )

I've set a setting in Google tools to reduce the frequency, I don't know if it will work as this is a setting from an old part of the old console, the new one does not have it.
Also, it can't be set for the whole domain, so in case of IMCDb there is with or without www, with or without https. I've set it just for the www+https version as it seems that Google rather uses that one.
We'll see if it improves the site speed in the evening.