Start by replacing the digitalWrite's for the slaveselectPin with direct port writes:
Say the pin in use is Port B, pin 2
PORTB = PORTB & B11111011; // pin2 low
SPI.transers
PORTB = PORTB | B00000100; // pin2 high
that will help a lot
Start by replacing the digitalWrite's for the slaveselectPin with direct port writes:
Say the pin in use is Port B, pin 2
PORTB = PORTB & B11111011; // pin2 low
SPI.transers
PORTB = PORTB | B00000100; // pin2 high
that will help a lot