do you have installed the latest driver for your graphic card?
Do you have tried to deactivate the GPU acceleration?
I forgot that the graphics card driver was also updated in the past week. I just unchecked the graphics accelerator in the preferences, closed and reopened Lr and there is no difference, the import dialog still acts exactly as described above.
I uninstalled and reinstalled Lr through Adobe CC. No improvement. I have another image editing program that has also become painfully slow to open and access old image files. I am thinking that the latest W 10 update did me in...
I would pull up the Windows Resource Monitor and see if it can give any ideas as to where your bottle necks are.
I am still on LR 2015.7 for my Win10 box. It moves right along.
Just for the record, the problem had nothing to do with either Lightroom or Windows 10. It seems that my backup hard drive, which is always connected for daily backups, blew it's brains out and would not allow Windows to access it. Since Lr "sees" that disk in its list of picture sources it tried to access the failed disk for quite a while during the import dialog before giving up. I found the problem by trying to access the disk just through windows explorer and it took a similar amount of time to finally give up and provide a message that the disk was inaccessible.
The second problem, Lr getting hung while going from the Library module to the Development module hasn't happened for a while but I doubt that it is related to the first problem.