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It sounds like your indexing locks haven’t cleared. ssh to your server and cd to var/locks in your Magento root. Run the folowing comand and the locks will clear and you should be able to reindex again .
sudo rm index_process_*.lock
I’d also have a look at your logs to see if you can find out what caused the index process to fail and the locks to stay in place.
Indeed when a large number of goods or attributes need to be saved, backend works very slow, as each time you save data the system re-indexes the goods through the entire catalog.
Asynchronous Re-indexing is a solution of this problem. When a product or category is saved it is not immediately re-indexed, but put into a queue. The queue is re-indexed in the background. This greatly speeds up the backend. This mechanism is implemented by using an extension of http://mirasvit.com/magento-extensions/magento-asynchronous-reindex.html