Roon's variable CPU usage

Just been monitoring CPU usage on my Roon install (in a VM).

Passing FLAC albums to Squeezebox via Tidal is low impact, but passing FLAC albums to Squeezebox from my own local library is really CPU intensive.

What’s the difference here? Other than the waveform calc that happens at the start of the track (I scan them as and when required), I can’t see what should tax the CPU so much in comparison to playing from Tidal.