VS code PowerShell extension intellisense is very slow
Since reinstalling VSCode didn't help, it's probably some issue with powershell.
Few things to consider.I used to experience some lagging with intellisense/ps loading due to some module folders were located on the network drive. I didn't find any better solution rather than copying all modules locally.
I had big issues with PS after upgrading from v3 to v5 on Win 7. Pretty much it stopped working. Downgrading back didn't help. Surprisingly it worked fine if I logged as different user, so I just deleted and recreated my profile and it helped. Log as different user and see if it work differently.
I use VSCode on my linux machine with powershell core (pwsh), as an ISE alternative. It works good, and I have same version on VSCode as you. Try to install powershell core on your machine and then switch VSCode to use it instead of 5.0https://docs.microsoft.com/en-us/powershell/scripting/core-powershell/vscode/using-vscode?view=powershell-6#using-a-specific-installed-version-of-powershell