Unfortunately, this has always been a problem with Marketo. The only thing you can really do to help this is to order your smart lists and use as little criteria as possible, or in a particular order that reduces the query time. Here is a link that may be helpful to you. Tips for Building Better Performing Smart Lists
Yes, this does not always work super well. Did you select Visible Columns or All Columns? All takes longer.
Also, you may be able to return to the list later or refresh to see Export Status. It may have already finished, but the UI never updated.
It's just the latency issue. If your smart list is based on complex logic, Marketo would take a bit longer to refine/process your leads. If you are exporting smart lists with option All Columns, Marketo exports data for all the fields of your each and every lead as a result this would increase export process time
Yes, I definitely agree with everyone saying not to export all columns; this could take hours. I've set up a variety of views based on my most common needs and select one of those before exporting to CSV.