El Capitan does not upload
My Macbook El Capitan doesn't upload new material or it is very slow. Have done a speed test but it doesn't give me an upload speed
MacBook (Retina, 12-inch, Early 2015), 10.11.16
if you posted from the computer with El Captain it did in fact upload.
see this article
Recommended settings for Wi-Fi routers and access points - Apple Support
check your computer for any 3rd party utility that cleans, keeps, snitches, sharks or otherwise "enhances" your OS X abilities and remove them as the developer instruction on their web pages, simply dragging to trash does not remove these apps and many of them cause interference with the normal operation of os X. Do the same for any Mac Anti-Virus solution for the same reasons. If you don't know what you have on your computer you may post an etrecheck report for further analysis.
www.etrecheck.com
past that you may need to contact your ISP to see if they intentionally throttle connections.