What repo are you using? You should be able to see the server name in the download window. Otherwise you can cancel your download, check your server name & restart. Synaptic will resume where you left off.
For me, in a somewhat remote spot, http://ftp.heanet.ie
give regular top speeds even though it is located continents away from me.
Oh, there's no need to delete the synaptic repo's from the list--just select the one you want. If I understand your post right, you deleted them.
I suggest you try a few repos to compare d/l speeds. Sometimes local servers are slower than remote ones.
Try these for starters.http://ftp.heanet.iehttp://ftp.nluug.nl
While you are playing to find a good repo you can set synaptic to keep your downloaded files til you are finished with them by opening;
Settings-Preferences-Files & select Leave all downloaded packages in the cache
Then reset later to remove them.
Hope that helps.
--and welcome to the forum