Why does the built-in Download Booster on Samsung not work with UC browser? Download Booster is a built-in function on Samsung devices, and it works great when downloading a file more than 30MB from the Play Store. However, it does not work when downloading a file via the UC browser.
Why?
It does not work / has limited support with Third-Party applications.

Have a look at the following article by Samsung on their homepage:
What is the Download Booster and how do I enable it on my Samsung Galaxy Alpha?

It seems like that the download booster is only available when downloading via the integrated Browser / Samsung Apps / Google Apps. 
See this image for the download browser description:

Thanks Samsung for providing this picture

The last sentence pretty much says that third-party applications might not work with the download booster.

You can also have a look at Even though Download booster feature on Galaxy S5 is enable, it does not work to download contents. What is the problem?.

I'm quoting the essential part:


  
  This feature supports Hypertext Transmission Protocol (HTTP) 1.1 only.
  So, It can be activated in Play Store, Samsung Apps, Samsung Video , YouTube, Play Movies, Play Games,   Internet(S-Browser), Chrome (ChromeBrowser), Baidu, and not all applications.
  The feature cannot be used with other protocols, such as HTTPS or FTP.
  So, it’s not supported regarding to attached files of E-mail , Streaming service like  Dropbox or Twitter