ST7735 display too slow - why ?

I'm using an 160x128 LCD based on the ST7735 connected using hardware SPI to a Nano 33 BLE.
And I find all screen operations too slow...
As an example, a fillScreen() takes about 1second !

I'm using the Adafruit GFX and ST7735 libraries downloaded from the IDE library manager.

What can I try ?