With the 3018 CNC systems, many I’ve seen have the maximum speed limited in the firmware to a very low number, like 8.3mm / sec. If you are using the GRBL driver, which varies power with speed, and you request a speed higher than the system is allowed to go, it lowers the power instead to compensate. You can fix this by either reducing your speed to the hardware limit, or increasing that limit in the firmware settings. The settings are here: https://github.com/gnea/grbl/wiki/Grbl-v1.1-Configuration#110-111-and-112--xyz-max-rate-mmmin
You can set them in Edit > Machine Settings in LightBurn.