Hello everyone, After many months of frustration trying to figure out why my 15 inch rMBP was experiencing very slow download speeds (despite being on the 100 Mbps fiber optic plan from Virgin), I think I finally managed to identify the culprit. When open, the PDF Expert app from Readdle is negatively affecting the download speed on my rMBP running OS X El Capitan (version 10.11.4 to be precise). It is rather unusual for such an app to cause so much trouble system-wide, but at least I am glad I managed to narrow down this very irritating problem which was driving me crazy past 2-3 months (I obviously blamed Virgin, the router, even OS X which is getting crappy these days, but apparently something completely different was at fault). I put a video on YouTube demonstrating the issue: . Basically, I get the promised 100 Mbps download speed when PDF Expert is not running, otherwise it's ~10-15 Mbps (it has been even lower than this a couple of days ago when I ran the same test). Has anybody else experienced this abnormal behaviour with PDF Expert? I would kindly ask anyone having the app to run the same test I did and to report back with their results. I will be sending this feedback to Readdle support as well. EDIT. I did reinstall the app in the meantime, but the same behaviour can be reproduced as in the video above.