I’ve tested this recently (this post week). Had a dns entry up and pointing to an nginx server for ~12 hours, zero requests. 17 seconds after the letsencrypt cert was issued, the floodgates opened. Over a dozen of requests per second.

I don't think it's necessarily specific to LE but rather to public certificate transparency logs. LE being free and easy to automate means it's very widely used these days, but if you theoretically go to a "pay" root CA and get a cert that covers thing.com and www.thing.com , the same probing will happen on the same time scale.