You may be missing a development configuration for your application.

In your angular.json navigate to architect -> build -> configurations and add these lines, below the definition of the production build:

"configurations": {
  "production": {
    // already there!
  },
  "development": { // this is new!
    "buildOptimizer": false,
    "optimization": false,
    "vendorChunk": true,
    "extractLicenses": false,
    "sourceMap": true,
    "namedChunks": true
  }
},
"defaultConfiguration": "production"

And to run the build in development mode just type the next command: ng build --configuration development.

Rerun update migrations

ng update @angular/cli --migrate-only --from=11.2.0

Explanation

Projects that have not used ng update to update to v12 are missing important migrations that switch some builder options to their default values in v11. Without this migration, the builder now assumes production configuration as the defaults were changed to represent production builds by default. An automatic migration that is run in ng update @angular/cli would set the affected options to their prior defaults, such that a project continues to behave the same. Without this migration however, builds are configured for production with the corresponding optimisations turned on, making development builds very slow.

Please use ng update @angular/cli --migrate-only --from=11.2.0 to apply the migrations or update using ng update @angular/core @angular/cli when still on v11.