RE: Dell Inspiron 15 3558 SLOW
Hi Lee KWAN
Your brand new computer should be lightning fast!
I'd suggest you check for latest windows updates are installed or failed. At left bottom of taskbar, click on windows icon, click on Settings,Update & security > Windows Update, and then select Check for updates. Install any available updates and restart your PC if required.
You can click on Advanced Options[windows update settings], and then View Your Updates History. Do you see any failed windows updates?