I am importing a 6,000 image Lightroom catalog off of my traveling laptop into my master 100,000 image desktop catalog and the importing process quickly progresses to the ~60% mark of "Checking for changed duplicate photos", when it then slows to a crawl -- 14 hours later, the import progress thermometer has barely moved. Both catalogs have recently been optimized. The importing desktop system is very high performance (Windows 10, i7-6700K @4.5GHz, 32GB RAM, SSDs). I am running Lr V2015.8 on both systems. The Lr process is taking ~15% of the system's CPU and 8GB of its RAM. It shouldn't take more than a half day (& counting) before one can even start working on a remote project's images! At a minimum, there should be a way for me to turn off the checking for duplicates, since I know that none exist!! In any case, any ideas on why importing is so slow and suggestions for speeding up this ridiculously slow process?
A lot of performance improvements have been made in the recent update release 6.10.
Please update to the latest release from the Help>>Updates menu and let us know if that helps.