I use l293d (parameters here)to drive two coreless motors (the coreless motor voltage is 3～5V, 8mm x 20mm). The L293D can be driven by Arduino, and I use 4 1.5V No. 5 batteries to power the motor power supply pins. But it feels that the speed of coreless motors is very low. Is my battery voltage too low?
That depends on your motor not the driver ic. Put 5v to the motor , there's your max speed. Hooking it to the Arduino only allows you to control it up to that speed.
Check your voltage across the battery pack when it's driving the motor, is your voltage dropping?