If someone suffering from the performance problem tries the beta (and I hope someone does), it would help me if you would capture and post the performance log. That information will help me understand where the bottlenecks are.
To get the log, run calibre-debug -g from a command prompt/box. Capture the output (there might be several hundred lines) and post it.
Also note that the best performance will come only with the second insertion. The first one will attempt to update the metadata, and I hope succeed.