This was the first (at least that I am aware of) topic reporting the bug:
That bug was my first thought when I saw @jeroen_v_s's post. However, this symptom doesn't match well with the conditions under which the bug reported at Data item properties in IDE 2.1 occur:
The only way that could fit is if the factor isn't actually time, but rather the interval between when a sketch on the C:\ drive is in use and when @jeroen_v_s switches to using a sketch stored on another drive?