Calendar slow performance loading contexts with large number of courses

From dmonllao:
DavidMonllao, [Oct 10, 2017 at 10:42:06 PM]
...have you noticed any performance issue being logged as admin in a course with the calendar block in screen?

I've loaded a real site database (around 5000 courses) and it takes me more than 1 minute to load moodle index

more info: it needs like 300.000 db queries to load the page; I am logged as admin and there is a calendar block in the frontpage; I've tried to track wtf is going on and I've seen that most queries are "SELECT * FROM mdl_context WHERE contextlevel = ? AND instanceid = ?"; debugging a bit more it seems that the problem comes from calendar_get_view and month_exporter, week_exporter and day_exporter; you can see the trace in https://pastebin.com/xkqzxANa

the sql dump I've used is https://drive.google.com/drive/u/1/folders/0B0DMrAQRleD4YnhfdXNFdm41dzA

I've just deleted calendar_month record from mdl_block_instances and the frontpage loads instantly (I couldn't do it using the web UI)