You may be right.
I've also heard people claiming that some of their provinces rebelled or at least had significant drops in loyalty for no apparent reason, and some thought it had to do with the very high taxes.
But again, this has never happened to me, and I always keep taxes on very high.

And yes, I guess the question whether the assimilation process is fast enough or not is very closely tied to your style of playing, and "fast/slow" is a very relative thing anyway