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.
it's not language you have to evaluate, but frameworks and app servers for clustering, data storage/retrieval etc available for the language.
you can use jython and use all the java enterprise technologies for high load system and do text parsing with python.
Python or Java for text processing (text mining, information retrieval, natural language processing)