It is because the speed is in a variabele.
The robot will stop every time to see if the variabele has been changed and resume when made the speed according to the actual value.
As far as I know there is no way to lock the variabeles, so when you want the speed in a variabele you will have to live with this.
If you want a smooth movement with no stops, change the speed in the instruction to a constant. (a fixed number)
In that way the robot knows what is coming.