I just opened a ticket, but posting also here for others to see if the issue is influencing their apps as well. But the issue is that if you have a Minimongo query with ordering opened on the client and then you subscribe to the documents of same collection, you might experience large slowdowns. Like many seconds slower performance of a subscription (time to ready) than you would have without that query. Really scary. Because in my case the issue is that these are two completely separate Blaze Components which are interacting at distance and making everything slower, just because one has a query (and its own subscription) opened.
cc @arunoda, if you have noticed this.