Cutting problem

If you are cutting at 50.00 mm/s this is 3000 mm/min.
I wouldn’t consider it slow for a diode laser engraver.
It’s very easy to overlook or accidentally shift the units.

"Plain cardboard cuts at low speed and 100% power."
This seems reasonable and unarguable.

"If I increase the speed, the machine cannot cut this ordinary cardboard."
This also happens. Is the plain cardboard and the ordinary cardboard from the same source and batch, or are they different?

"What is the cause of the power loss?"
I don’t follow. Were you working with known material and known good cut settings?

"It’s thin cardstock and I was cutting it at a much higher speed and lower power before. Help me!"
Some light colored products reflect blue light quite well and prevent the laser from removing material. This can be startling and unanticipated after working with even slightly darker materials.
Under Laser Tools, there is a Material test to help develop new settings.
You may have to develop new settings for each new material. You can also go back to previous settings and test material you already have experience with to confirm that your laser engraver is performing as expected.