Hello,
I'm using the L293D chip to control a 12v DC motor and I'm connecting 12V battery to the L293D... but when I'm sending value of 255 the motor rotates much slower than when I connect the battery directly to the motor... what can I do?
Most likely, the old and very inefficient L293D can't handle the stall or running current required by your motors.
Post a link to the motor data sheet or product page, or measure the winding resistance and tell us what that is.
In any case, you will be much better off with a modern, efficient MOSFET motor driver like those from Pololu.
This is the motor:
https://www.servocity.com/files/index/download/id/1470165364/
Did you notice the stall current on that sheet?
How does that value compare with the current handling capability of the L293D?