Newbie problems - cut result is not consistent with speed setting

Hello! Been trying to familiarize myself with a new OMtech 50W CO2 laser. Lightburn seems to be a way to go, but I find it odd that ramping up or down some burn settings has unexpected results. I have made a pattern with fixed power and varied only speed for both engraving and cutting tests.

While engraving, I can see that slowing down the speed results in deeper engraving until it suddenly turns very shallow and then really deep. At cutting, the edge may be cut cleanly, then not at all and suddenly cut again, depending on the speed level used.

Another thing that I noted is that in the Y direction, the cut lines on the 4mm acrylic sheet tend to be “dotted”, while in the X direction, the cut is clean. Also, a curved cut has steps that may result from Y movement.

Is this a Ruida (Rykon) related issue, a mechanical issue or something that I should look at Lightburn?