Download speed of an app at the App Store? I think the title is self-descriptive, but I'll describe it more precisely here: What I mean by "Download Speed of an app" is when you click in the download button of an application, at the Purchases tab we'll see the app downloading, like this:



Where can I see the download speed of OS X El Capitan, in the case of this image?



This might have been asked before, but I don't find it anywhere. 
I tried clicking the progress bar, but nothing is displayed, the only thing that the Purchases tab displays is the app, downloaded size, and time remaining.

This question takes me to another question: How do I know what's all that's being downloaded? I ask this because I feel the download speed is very slow sometimes, and I'd like to know what are the apps that are downloading stuff...
The download speed on the App Store depends on two things: speed and size. If you have a DSL connection (the average connection is around 128kbps to 3000kbps (3mbps)), it is going to be very slow to download an app. If you have fiber optic internet (which can range from 10mbps to a Gigabit) it is going to be a hell of a lot faster. 

If you are downloading El Capitan, which is 6.2GB, and you are downloading on a 5mbps connection, then it will take almost 3 hours to download. 

You should take a Speedtest here and see what your speed is, you can also see if you are getting what you paid for. Go to this link and input your download speed here.

I have found that if you have inputted DNS servers in System Preferences, remove the DNS servers and that should make it a little faster.

Happy downloading!

Cheers 

EDIT: If you want to see how fast it is downloading, try Rubbernet which shows how much bandwidth a process or an app is using. Another app (which I use), is iStat Menus. It only shows processes that are using most of the bandwidth. The process that you are looking for is "storedownloadd" which downloads apps on the App Store. Sorry for misunderstanding you.