Ubicom, formerly known as SCENIX, made (I don't think they do it any longer) a series of "SX" chips which were essentially high-speed one-clock versions of the small PIC chips. At 100 MHz, these were pretty useful. Their flash process wouldn't keep up with their CPU core, so they copy the FLASH content into SRAM each time the chip is reset and execute it from SRAM.

It's hard for chip makers to integrate FLASH and fast SRAM technology. It's also hard to make fast FLASH. However, if you want your MCU to run really FAST, you have to solve one problem or the other.

RE