Speed setting, I’m a little confused by adding a higher speed and it taking longer to complete the project

The gotcha for engraving happens in the overscan area on either side of the thing you’re engraving. In order to maintain a constant speed across the engraving, the laser head must accelerate from a dead stop off to one side, cross the engraving area, then decelerate to a dead stop on the other side.
Then it repeats in the other direction, over and over again.
The faster it moves, the longer it takes to speed up and slow down. Ask it to go crazy fast and it’ll spend most of the day outside of the engraving area, getting ready to zip across at top speed.
If you restrict the overscan area enough, then the hardware can’t get up to speed and will produce overburned areas along the sides.
If you increase the acceleration settings enough, the motors will stall because they can’t produce enough torque to get the laser head up to speed.
If you increase the motor current to increase the torque, the motors will become scary hot or the drivers will burn out.
The controller settings limit the actual top speed, but the laser power and material limit the practical top speed to a much lower value.
Find a speed / power combination that produces the result you want and run with it …