Is it still slow after you have loaded an assembly and saved to the new SW2015 file format?
There are major changes to the file format in SW2015 that should lead to faster load times once your files are saved as SW2015 files.
Thank you for answer
Were stored at SolidWorks2015, it will take some time, even open in 2015 version of the data format. Multiples of the time is now consuming as compared to 2014.