The tips here are all valid but for me the biggest gain was when I analyzed my bundle js file, first check the file size because the parsing at the launch of your application is very important for startup speed. Then analyze your bundle with source-map-explorer to look if there are bloated librarys/modules that you don’t need for the first screen.
It takes time to optimize an app and just to compile and judge a starter kit that is not optimized might not show the whole picture of ionics possibilities.