When you add more files, Spotlight creates and updates its indexes so, it will be easier when you search for files on your mac. The downside is, it always perform this and slows your computer. To speed things up, you can exclude folders, files, for indexing:
System Preferences->Spotlight or, Launchpad and click the Preferences icon.
Click Search Results tab, and uncheck all that you don’t usually search for.
Then, click the Privacy tab and click the plus (+) icon and choose folders you want to exclude for Spotlight to index. If you exclude more folders, the system will be much faster.
Need For Speed!