The problem are the servers.
They are all in China and access to them isn't that fast in countries which are far away from that.
I'm in Germany and I have the same problem.
If you use the apt-get command line tool instead of the graphical store,it can be fixed very easy.
Run the following command: sudo gedit /etc/apt/sources.list
Now replace the Mirror URL (beginning with http:// and ending with the first space after the http://) with the one of the mirror nearest to you.
You can find the list here https://www.deepin.org/en/mirrors/packages/
Now save it and use the fast apt-get
It is very unlikely that this will work with the Deepin store,too.
I think it doesn't use the specified mirror but their own servers which are all at one location.

Thank you very much nipos. I did what you said and it worked.

Just a tip: you don't need to edit it manually, you can do it from GUI going in control panel -> Update -> Change Configurations (options?) -> Change Mirror. It will show you a list of all mirror available, you can use the "sped test" button to test the speed of all of them too.
But please note that for some reason some mirrors don't have all packages of deepin, so some of then can return some error when trying to install some apps.

Theoretically your way should work too.
But I noticed that this doesn't change anything and another guy reported that,too,but I don't know the thread anymore.

update kernel 4.13.5 use program ukuu  update

Basically, you can choose a nearer mirror or install/update via command in terminal.

1. It will fetch the correct package from official site if the package can be found in the mirror site.  (Only If you change the mirror in control-center instead of edit /etc/apt/source.list.)

2. The official site is a custom CDN , but not very well out of China.

I also had to change the mirror manually using your method, so, apparently there is a bug.