I'm soon to start on a new project where I am going to do lots of text processing tasks like searching, categorization/classifying, clustering, and so on. 
There's going to be a huge amount of documents that need to be processed; probably millions of documents. After the initial processing, it also has to be able to be updated daily with multiple new documents.
Can I use Python to do this, or is Python too slow? Is it best to use Java?
If possible, I would prefer Python since that's what I have been using lately. Plus, I would finish the coding part much faster. But it all depends on Python's speed. I have used Python for some small scale text processing tasks with only a couple of thousand documents, but I am not sure how well it scales up.
Just write it, the biggest flaw in programming people have is premature optimization. Work on a project, write it out and get it working. Then go back and fix the bugs and ensure that its optimized. There are going to be a number of people harping on about speed of x vs y and y is better than x but at the end of a day its just a language. Its not what a language is but how it does it.
Python or Java for text processing (text mining, information retrieval, natural language processing)