I was playing around one day, measuring MCUs with resonators and crystals, and found that resonators have a relatively long warm-up period, perhaps 8 to 10 minutes! Some speed up as they warm up, and others slow down. The MCUs in the charts below all had 16MHz clocks, I had a timer programmed to output 1MHz and that was what I measured. So the y-axis is Hz, the x-axis is ms.
