fm:
If you look in the project's wiki you will see that the 2 wire SR version of the library is about 4,5 times faster than the stock parallel LiquidCrystal an almos 40 times faster than the I2C versio.
Just don't try to convince GrumpyMike.... 
Tack,
GrumpyMike argued and argued that the display update rate to the LCD was in no
way impacted by the interface used to talk to it. He flatly didn't believe
our transfer numbers and update rates vs 4bit and i2c.
And then for costs, he said i2c was just as effective and the
same cost as using a shift register.
I showed him how you could build a HC595 based board for $1 USD QTY 1
ordering parts from tayda electronics (20 cent 595s and cheap strip boards) and to show me where I could
find 20 cent PCF8574s. He never replied....
To see the update rate, run the LCDiSpeed sketch I wrote.
It is included in the examples.
You will have to modify the i2c constructor for your i2c board.
--- bill