Hello. This thread has become a catch-all for various problems.
Indexing should happen only once. When it happens, you do see a popup notification with a progress bar (v1.8.9+).
We are aware that there should be a relatively high (1 CPU core utilized) during and shortly after startup while Obsidian is reviewing of how your vault looks like (if anything has changed). The time it takes is proportional to the number of files in your vault.
If you experience high CPU usage in other circumstances you need to enable Restricted mode, use the default theme and restart Obsidian. If it happens again under these conditions, please open a bug report and follow the bug report template.