So what is happening is you’re splitting your movement between diagonal left and diagonal right, canceling each other out. So if you went diagonal left, that’s 50% acceleration left, and 50% speed forward, same with right. So you’re nullifying your left acceleration with your right acceleration and vice versa when you bounce back and forth like that, causing your forward motion to be 50% speed due to the nullification of the other acceleration.
If you’d like to travel forward at the same speed even with sideways movement, you need to add forward movement input along with your left/right input! 
maybe multiply it by 2 while moving left/right.