MacOS Mojave: Always indexing, the whole OS is too slow
OK, here's how I solved the issue, I hope this will be useful for anybody with similar issues:
I've done some search and found that I needed somehow to fix the indexing files used by spotlight, there are some sudo commands you can use to delete the old index and start all over again, I didn't follow that, I did the following:


Open System Preferences -> Spotlight 
Click on Privacy tab.
Open A finder window, navigate to the top most level, drag and drop the "Macintosh HD" icon to the spotlight excluded items in the privacy tab, that way Spotlight will not perform any indexing for the entire hard disk.
Then click on the "Macintosh HD" item in the list and click the minus sign button to remove it, this way Spotlight will start indexing from scratch and hopefully succeeds.