Yes you may use digitalRead inside the ISR, but if you're finding it too slow you might have to ready the binary data directly from the register.
Don't forget to declare any variables used by the ISR as volatile.
Also, at the end of your setup() method, you'll want to call sei(). This will set the global interrupt enable bit. In case you ever want to reset the bit, (thereby turning off interrupts), you may use cli().