This needs to be supported by the controller to work. Are you sure the S1 supports this feature?
Is this for a vector graphic or image? Unexpected slowdowns could be possibly explained by things in the design not obvious. The skipping lines should never occur. Either the machine is losing steps or possibly a firmware issue.
How do you know what gcode has to be entered? Is this an xTool requirement? If so, then that’s more about the idiosyncrasies of the machine.