Delay when accessing file sharing (particularly xls/xlsx/xlsm ppt/pptx i.e. office file extention)

Using ZPA , when accessing files on network file sharing , it occurs delay.
e.g.
・Making new file with xlsx extention takes 6 seconds
・Changing file name takes 2-3 seconds
・Saving xlsx file (about 7KB) takes 2-3 seconds
・Opening xlsx file with links to other xlsx files takes 40-50 seconds (file size: 937KB)

File sharing privides with on-premises file server using NetApp and SMB ver 3.0.

I tried with some internet access , result was same.
I also checked using IPsec-VPN (FortiClient and FortiGate) , result was same. (Strictly said , with IPsec-VPN was little bit faster then with ZPA, 1-1.5 seconds).

Never happens delay with Ethenet connection.
Also, never happens delay any other services (e.g. any SaaS like Teams,Exchange and SharePoint ,any other Private Access applications) , so internet is not slow.

I think accessing file sharing via internet is causing these delay.
Are there anyone experiences same issue?

I want to solve this problem.