My pc is very bad that's why it draws fps or do I have a dilay when I play fortnite in the background on spotify and listen to music so on pc?
If you have 1kb of RAM
Spotify shouldn't have a significant impact on your performance. If the PC is that weak, it wouldn't be suitable for Fortnite anyway.
For a rough assessment:
Spotify is currently using 0.2% of my CPU power and 140MB of RAM.