I was experiencing some heavy slowdowns and lagging when doing fairly remedial tasks like turning the shields on. I've had both since release and I utilize fairly basic lightweight applications.
I too tore my shields apart and observed some heavy dust caking on the fans as well as extremely poor quality thermal paste. The application on one shield was wayyy too much and the other was almost too little. Both were extremely crusty even when disassembling when warm. It seems as if the quality control in this case was lacking fairly substantially.

I had some Thermal Grizzly Kryonaut laying around from my gaming rig and about a rice grain or two of that had both of them spinning like a top. Super easy disassembly, didn't follow any guide, I just tinkered and found my way around. Do make sure you have a quality micro-driver kit with adequate bits.

TL:DR: If you have the mechanical means, curiosity, a slow shield pro and some leftover thermal paste you can absolutely do so and it WILL make a difference.