It was suggested that i had "reached the practical limits of what you can do on a Mega." But on the uno it was only marginally better.
Since the Mega has more code space, and more memory, I'd expect it to wok better on the Mega than the UNO.
Serial.begin(9600);
The stone age ended a long time ago. Pick up the pace!
// if sharing pins, you'll need to fix the directions of the touchscreen pins
//pinMode(XP, OUTPUT);
pinMode(XM, OUTPUT);
pinMode(YP, OUTPUT);
//pinMode(YM, OUTPUT);
On every pass through loop()? I doubt it.
tft.setCursor(0, 0);
tft.fillScreen(BLACK);
tft.setTextColor(GREEN);
tft.setTextSize(5);
tft.println("Waiting");
On every pass through loop()? Again, I doubt it.