CSV uploads generally upload quickly, but the amount of data you are uploading will affect the speed at which we can process it. In addition, if you've uploaded multiple CSVs in a row, this will slow things down as well.
Uploading a CSV that is more than 10k rows is not advised, if you need to import more than that splitting the import up into smaller chunks is your best bet!
The best thing to do when you are worried about a CSV uploading is to check its process under Settings > Imports & Exports. We will show you how far into the upload we are, and if we're still processing it you should hold off on uploading another one.