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 very difficult to answer questions like this without trying. So why don't you

Figure out what would be a difficult operation
Implement that (and I mean the simplest, quickest hack that you can make work)
Run it with a lot of data, and see how long it takes
Figure out if it's too slow

I've done this in the past and it's really the way to see if something performs well enough for something.
Python or Java for text processing (text mining, information retrieval, natural language processing)