I have face issue for file and folder accessing
Hi and welcome to the forum. 
You’ll need to provide a bit more information than that. Let’s start with the basics.
What is your NAS model?
What is the QTS firmware version installed?
Are you using HDDs or SSDs?
Are any warnings or errors shown in your NAS system log?
How are you accessing your files? Via a network share? Via a website application (like File Station)? Via VPN?
nas model - h1886 xu rp r2
firmware version- quts hero h5.2.9.3410
Storage HDD,
No warning
Network Share
You still have provided us with very little information. What do you mean by the access is slow? Please describe your issue in more detail.
What model and size hard drives do you have?
How many?
What is the drive configuration?
Have you tried the basic step of shutting down and restarting?
Have you run any drive tests?
What changed 16 days ago?
What applications are you running?
Most important, have you backed up any important data?