A large collection of duplicate Photos, iTunes (now Music) files, etc. take a huge chunk of the storage space. If you are running low on space, having too many duplicate system files could be the reason.
Unwanted or unused applications stored in your Mac at different locations occupy space and slow down Mac.
Keeping a collection of large and duplicate photos that may be stored in different folders with different names is probably the reason why your Mac is running slow.
Large-sized video files can use up a lot of disk space on your Mac SSD. This can make your Mac act in an unusual manner.
Having duplicate documents stored on your Mac at different locations with different names could also be the reason for start-up disk’s sluggish performance.
All kinds of duplicate files of Aperture, iWork, iMovie, Photos and iTunes occupy a great amount of space on your Mac system and lead it to a slow performance.