Is there any way to speed up dropping a user when they have a large number of tables etc?

Jonah is correct. But oddly, there are some bugs on Tablespace Quotas that slow down drop user. If you remove any tablespace quotas first then quite often the drop user is faster. This is particularly evident if dropping many users. So you could try that.

In addition to handling the normal UNDO transactional semantics of the DROP, I don't believe there's much you can do, because Oracle has to internally perform removal of all dependency information and invalidate any/all objects that reference ones you are dropping.

Some people say you can cheat by dropping/truncating tables/tablespaces/data files first, but the sum of time spent is generally equivalent and it's more complex and error-prone.

Ok - it's a longshot, but trying REALLY hard to think outside the box...

If your db in question has only the 1 major schema, shutdown your database remove the files and create a new one. It's pretty drastic I know, but otherwise I can't think of anything to add to Jonah's answer.