Hi Oliver,
I managed to add support for Arduino ATtiny85 hardware SPI. I tested it with an SSD1306 OLED controller (Adafruit 128x32) and it seems to be working OK. As predicted, it spares some CPU time and program memory.
The mod is attached. Feel free to include it into future releases, as is or modified.
Part of the credit goes to Nick Gammon, thanks to the code he posted there: Faking SPI with Attiny85 - Programming Questions - Arduino Forum
Danke schön for your library. 
ATtiny85_hw_spi_mod.zip (16.1 KB)