Consider averaging those measurements and printing them at the end if you're concerned about a serial bottleneck.
Also, when you're looking at speed issues, Arduino type hardware is cheap; if you've removed the most egregious time sinks and it's not enough, consider whether you can purchase your way out of your problem with something like a Teensy.
Well I'd like to know if there are fluctuations, that gets lost with low passing. A middle road could be printing the avg plus the lowest/highest values. Then scaling the number of measurements doesn't get increasingly stressful on the Serial output. But again, for me I'd feel like I'm missing out on insights.
+1 on buying myself out of performance issues I hadn't considered that yet and you're absolutely right.
I do that a lot in all sorts of places in my code for various reasons. I also usually have 3 LEDs on any project for this kind of testing, although with Arduino the serial monitor makes doing this less necessary than with PICs. Typically I will toggle a pin to see if a bit of code is reaching a certain point when something doesn't work as I expect, or toggle it in 2 places to measure the time between 2 points. I am guessing you don't have an oscilloscope. I suggest getting one, you will find loads of uses for it. Once you can see what a pin is doing, or the time difference between what one pin and another pin, or see serial data as actual bits rather than what the serial monitor tells you, you will find debugging a lot easier.
Ah, the LEDs trick! Yes, I had forgotten about that. I can see how it can be a valuable strategy although I imagine that it can be somewhat tedious to need to set it up, both in code and requiring an LED somewhere. I do have an oscilloscope but I'm just getting into that domain. I got myself a cheap one which as I understand it isn't completely helpless. I'll try the digital pin flipping and reading it with the scope. Do scopes generally have the ability to count the number of triggers in a given time period?