Yes, the results are (in my experience) much better if you use speeds slow enough to burn all of the model and with sufficient power to leave a mark and yet not so powerful that the edges of the workpiece are scorched wherever the laser beam touches. I aim for smokeless engraving and then I know that I have the right power settings.
Laser diodes are components that can wear out with use. Running the laser at high powers constantly will shorten the life of the diode. The clue is always going to be the manufacturer’s claimed diode life. When they claim 10,000 hours of use, I very much doubt that this figure is at 100% power output. Perhaps some person with knowledge of this subject will be able to clarify the point.
I see no point in using a laser beam, which is very fine, if the end result is a blurry image. One of my projects here was a birdhouse. I needed to laser engrave feathers and you can see how they turned out here:
The wood was a soft cedar that scorches easily. The first image is a view of the feathers and using too much power would have spoiled them. Too much speed will not permit the laser to burn the image everywhere. I see no reason to save five minutes if the end result is worse than I wanted. The text of the wedding piece was important so it had to be legible and because it was bold text, I wanted to take care that it was sharp without the over-burn that would make it less pleasant to look at and potentially illegible.
Happily, LightBurn has the facility to specify travel further than the edge of the image so that the laser beam burns to the edge of the image and then turns itself off, while the laser head is slowing down. From its stopped position the laser head accelerates up to the image edge and then turns on. Fantastic software that will do all it can to give you great results. You could try to reduce the power and speed you are using and see if your burns look sharper.