Tnx fr info. I've used EA displays in the past and they are of good quality. I have no speed issues, i.e. response time, with the LCD itself.
Also the Teensy 3.5 @ 48 MHz appear to be more then able to handle the tasks. Nice thing of the display you pointed at is that it also has a SPI I/F
That saves a lot of I/O pin's, something to keep in mind for future projects.
I suspect that what is happening at higher clock speeds that the timing of the control lines of the display do not stay within specs of the controller chip.
My display is of unknown origin obtained many moons ago on some flea-market and had been gathering dust for years.
That suspicion is only based on earlier experience with the SPI interface and I have not really investigated it.
There are still lots of other and bigger fish to fry.
And just FYI and amusement here are some pictures on what I'm up to.
73, Pim, PA2PIM