I have this problem since I started using X plane (10 and now 11).
Everytime I load up the program and start a flight (with any plane: default, pay, free, etc) on any scenery (pay, default, free) every program that is downloading something slow down.
I have a pretty decent internet connection (cable via powerlink, already checked that) with a top speed of 45 Mb/s (Download) and 10 Mb/s (upload).
Example: I was downloading a file at 2/3 MB/s without X Plane and when I opened it the speed went down to 29 kb/s.
In this months I tried everything: lowering settings, reinstalling X-Plane (both 10 and 11, even the demos), reinstalling Windows (8, 8.1 and 10), reinstalling drivers, changin to wi-fi. Nothing worked, I still have this problem and I am pretty upset about it because no one was able to help me (and I have to say it no one was willing to help me).
My computer: i7 3770K @ 4.3 Ghz, 16 Gb RAM DDR3, GTX 780 Ti, X plane is on a SSD (Samsung 840 EVO 500 Gb).
Sorry for any english mistake, it's not my first language.
Thanks for help