I've searched for this issue and I found these three useful GitHub links that can help you:

• Bug in VSCode Angular Language Service with autocomplete (solution included) : in here the asker founded that everything works correctly if strict typing is disabled within the plugin.
• @angular/language-service slows down VSCode autocomplete on every keystroke : in order to find out why is this issue appearing, you can open the Output tab in VSCode, and check any warning or errors that can give you a clue of what is going on (discussed also in the 3rd link).
• autocomplete not working on windows : In this link it's recommended that your project's tsconfig.json is correctly set up and that the Angular Language Service can find all necessary files (discussed also in the 2nd link).

Note that there are also many other related links for your problem (especially in the 2nd link).

I hope it helps you!