Download, as in obtain from a site hosting the content? That has nothing to do with what kind of files they are, it's a matter of your internet bandwidth (sounds fine), that of the source/server and how busy it is at the moment, and some variables in between. But unless your hard drive is full or very nearly so it should take exactly the same amount of time to download two 6 MB files of different file types from the same source, all other things being equal.
Just download, meaning that the end result is a file sitting on your Desktop or in your Downloads folder (or some other place) that wasn't there before, not that it gets accessed, attempted to run, or put away anywhere else automatically. Web browsers drive me crazy when they try to predict what we might want to do with a download and "helpfully" try to do it for us.
Rar files (like zips and 7zs) do not belong anywhere near your TS3 user game folder. They need to be decompressed first and the package files inserted into Mods\Packages\(maybe some subfolder). If any are sims3packs, then those would need to be Launcher installed. Some designers offer both for the same content so the player can decide which format to use. But TS3 cannot read or work with compressed files like rars, zips, and 7zs. The game would just choke on those as it attempts to start up.
Or did you really mean that your game takes too long to startup once you have added certain types of content? That's of course a very different matter.
