Sandwurm bestiary progression is very slow

I've noticed that my progression on Sandwurm bestiary seems to be really slow. I'm only getting about 1.5% per wurm, which means that to reach 100% I would have to go through the desert between 30 and 40 times. Comparatively, Wyverns and Manticores in the same area give something closer to 5% progression per creature, so I'd only have to go through the desert around 10 times or even less.

I'm also using wurm-killing optimal gear to do this : Silver Cryo hammer in the main hand (two of it's weaknesses) with a light slashing weapon in the off-hand to be able to break out.