You should not have this line
Motor_1.runSpeed();
inside any IF statement. Just put it at the end of loop() so it gets called every time loop() repeats. And make sure that loop() can repeat much faster than the fastest step rate that you require.
The same for the other motor.
The way your program is written now runSpeed() will only be called once for every time that a 'B' is received.
That LCD code will also slow things down. The LCD does not need to be updated on every iteration of loop().
Is that a program you wrote yourself or one you found somewhere? Either way it is a bit of a dog's breakfast.
Serial.parseInt() is also slow. Have a look at the examples in Serial Input Basics - simple reliable ways to receive data.
Separate the business of receiving data from the business of setting the AccelStepper parameters and that from the business of actually making the motors move.
If you want some action to happen while a button or key is pressed send a message when the key is pressed and send a separate message when it is released. For example send 'B' when it is pressed and 'b' when it is released.
...R