Have you tried the "Parallel processing extension"? Once installed you will notice a flag in the "process document" operator, that allows parallelization of the vector creation. If you have a multi-core machine that should speed up quite a bit things.
One thing I noticed though is that if you have big data sets it might saturate all your resources and it did happen to me that the process crashed (it might be just because I have a very old machine with 2GB and 4 cores).
Anyhow.. to avoid that I found much more useful to work on data coming from a DB... So what I normally do is:
Load my data into a DB table using RapidMiner
Use the StreamDB operator to feed things into the parallelized "process documents"
write my results on another DB table.
Hope this helps