Very interesting, Ismael... and quite uncanny timing, since I was looking at shutter speed testers just this morning

Thus far, I've been using Audacity to record the sound of a shutter, looking at the recorded waves and figuring out - as best I can - the time between curtains or leaves opening and closing... but the audio characteristics of each shutter differ hugely, and at faster speeds it can be really difficult to differentiate shutter sounds from the rest of the mechanical noises. Tools like the one you've bought - which use a light sensor with a narrow field of view - are capable of much greater accuracy. I'm tempted to get hold of one myself, but I could really do without the expense right now

Perhaps once I've sold off one or two items I'm no longer using...