You may not connect a pin from a Arduino Mega 2560 board directly to a pin of a 3.3V sensor !
Changing the AD0 to low only changes the I2C interface of the sensor. The accelerator and gyro keep working. There is no delay.
The software I2C is not much slower than hardware I2C. In some cases it is faster.
Suppose you have four sensors with four I2C addresses on the I2C bus. Then you have to use the Wire library to collect the data one by one. If you use AD0 to select a sensor or use a extra software I2C bus, then you still have to collect the data one by one. You won't even know the difference.
The Wire.endTransmission() and Wire.requestFrom() wait until the I2C session has finished. The Arduino Wire library uses a interrupt that handles most of the I2C functionality. But since it waits until the I2C session has finished, a software I2C library is almost as fast.