Slow performance when opening MS office files on CIFS share from Windows10


We have a NetApp FAS2220 running Ontap 8.1.4P1 7-Mode, and we're now testing it on Windows 10.
We have a slow performance problem when opening MS office files on CIFS share from Win10.

The test file is Excel 2010 document which size is 1.5MB.
Antivirus software and other management software is all the same between Windows 7 and 10.

Test result is as follows:

1. From inside our office network (via Wifi)

When opening CIFS share folder using Windows Explorer, it takes alittle bit more time to display all files in the folder.
Average time to open the test file is below.
Win7 ---> 2-3 sec
Win10---> 3-4 sec

2. From outside our office network (via SSTP-VPN over 4G)

The performance is getting slower. The average time to open the file is below.
Win7 ---> 10-15 sec
Win10---> 25-30 sec
#You can be frustrated when accessing the file from outside office network.

This problem seems to occur only on office documents, there are no slow down when accessing other file type, such as PDFs.
And we're testing Windonws10 using MS Surface pro and HP Laptop, but the result is the same.

Have you seen similar problem before and know any solution to speed up the performance?

Thank you.