No experience with tft but you do a lot of updates that don't seem necessary. E.g only update the relevant part of the screen if filtermode changes, not every time that you ho through loop().
Out of curiosity, what are those last two elses suposed to do?
if (Encoderbutton == LOW) {
// encoder button was pressed, increment menu position
MenuSelect++;
if (MenuSelect >= 8) {
MenuSelect = 0;
}
else (MenuSelect);
}
else (MenuSelect);{
}
You have a few more like that.