It seems that some Quarry queries cannot be completed due to an overload of pending requests that generate a global slowdown for all the executions (read this topic: https://www.mediawiki.org/wiki/Topic:Tt2l70vyi14eafza). Please take into account a reset of the requests queue as a possible solution. Thanks in advance.
Have a look also on the Quarry maintanance request that was posted during the 2016 Community Wishlist Survey, where there are old reports similar to this one: https://meta.wikimedia.org/wiki/2016_Community_Wishlist_Survey/Categories/Bots_and_gadgets#Quarry_maintenance
I solved the "issue": there wasn't a real slowdown actually, but a simple bad implementation of queries (see latest replies on https://www.mediawiki.org/wiki/Topic:Tt2l70vyi14eafza). We can surely close this topic without regrets.