You need to look up "direct port manipulation."
Each pin on the board is connected to an 8-bit group called a port. You could assign the appropriate port an 8-bit value. For example, PORTD are pins 0 through 7.
So after you configure the pins for output,
PORTD = 0x55;
That would cause the pins to alternate HIGH / LOW. (0101 0101)
If you look at the schematic for the Uno, you'll see the pins in the ATmega328 have a "Pxy" designator. X is the letter of the port and Y is the bit of that port.
The downside to using PORTD is that it's also Pins 0/1 which are responsible for USB/Serial communication.