It took some time, but I finally found a library for the SSD1331 OLED which makes use of the hardware SPI called "ssd1306". This library made use of the VSPI, leaving HSPI free for other peripherals if needed. Note for readers: the V in VSPI and H in HSPI doesn't mean Virtual or Hardware, they have no meaning. The display writing rate was significantly faster than software SPI like the one used in SSD13XX library or the Adafruit SSD1331 library (I couldn't get the hardware SPI work with the Adafruit library). The maximum refresh rate for displaying full screen 16bit images was left to be determined.

Fig. 2 - Arduino sketch window of the "ssd1306" library demo for SSD1331

Fig. 3 - SSD1331 OLED displaying a bitmap through ESP32's hardware SPI