You may want to turn off “Determining Gapless Playback Information” in iTunes, it is slowing down your system whenever you are adding songs.
Apple does not provide an option to turn this off in iTunes. Try the method suggested in the 2nd page of this post: iTunes – Determining Gapless Playback Information.
You may also have to turn on the Crossfade option for the above workaround to have an impact: Apple Support:What is Gapless Playback?
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