After an initial calculation, the results are cached and thus can be reproduced much faster; when you close openLCA, the cache is deleted, therefore the first calculation of a given system takes more time. 10 minutes is quite long though. Check that you do not see "you can make the calculation faster" in the welcome screen in openLCA (the organge ribbon); if you see it, click on "learn more" to download the fast numerical libraries. And allocate sufficient memory (> 8GB for ecoinvent APOS, e.g.).