Optimizing takes long time on TREX 7.0

Hi

Optimizing is taking a long time on our TREX 7.0 Production instance. The DEV and QAS TREX instances don't seem to have the same problem, with the same amount of documents for each. Optimizing on DEV/QAS takes around 1 minute, but Production was taking 5 minutes and now it's even slower, taking over 30 minutes.

Any ideas what I can check to find out why it is so much slower?

Also.. does a document need to be optimized before it shows up in the TREX search, or is indexing enough??

Many thanks

Jane