You are using the wrong approach. You are treating the colour TFT like a monochrome OLED.
I ran your code on a Nucleo last night. You are drawing a new frame into the SRAM buffer. Then blitting the whole buffer in one go.
Yes, you can do the blitting faster. But do you really want a black and white picture on a colour display?
I suggest that you use the regular Adafruit_ST7735 library. Try all the supplied examples.
Bodmer is faster. There is probably a ST7735_PDQ library. This would be pretty damn quick too.
David.