I think this particular forum is not the appropriate one for your question. This forum is about scripting InDesign.
Nevertheless: File size could relate to speed. There are various factors to consider.
Where exactly, at what point of your workflow would you like to measure speed?
Placing images?
Exporting images?
Printing images?
Walking through pages where images are placed?
Previewing images?
…
Would you also consider other file types like PSD or EPS for images?
Uwe