Comparing 20 & 40 teeth blades versus 80 & 100 teeth blades:
Most brush cutter blades spin so fast that ONLY the very outer tips of the blade teeth do the actual cutting (in most applications). The deeper gaps of larger teeth blade types (like the 20 of 40 teeth) don’t even have cutting carbide down in the gaps so deep teeth are not really beneficial.
From our experience, only when the material being cut is harder wood base (tumble weed type) or more bendable as in the case with hedges (hedge limbs want to bend instead of be cut) do the stems and branches of the foliage being cut actually sink down into the deeper teeth gaps of the 40 teeth blade types you mention. Think of the standard brush blade that is spinning (super fast) at 6,000 to 10,000 rpms as seeming like a round disk with just one continuous sharp leading edge, like a sharp edged saucer due to such speed. If a hedge limb/stem can sink down half an inch (example) into the 3/4? deep gaps between blade teeth, that says the application is NOT being cut well out at the sharp tips of the teeth, and is being grabbed and torn more than cut. I hope you can picture what I am describing.
So larger deep grooved cutting teeth like you find with typical 40 teeth blades most often do not come into play except in rare cases and those cases indicate inefficiency as no cutting edge exists in the deep teeth gaps. The depth of the teeth are simply not accessed by most brush, stems, limbs, brambles, and branches.
40 teeth type blades get especially stuck in thicker tree limbs as that is when the depth of the teeth comes into play most noticeably, but teeth depth in such an application works against efficiency and only adds friction. So, every 5 seconds you find larger teeth blades (like a 20 or 40 tooth) get stuck in the tree branch and completely seize, which is very time consuming. The only time larger (but fewer) teeth apply beneficially is when cutting in thin softer grass like brush that tears easily, if not cut. In such a case you can see the grass actually being grabbed and pulled out at the roots by the big tooth blade or torn off (jagged), not cut properly.
So it makes sense efficiency-wise to have more aggregate sharp teeth with less deep gaps, yielding more carbide at work, which is the case with our blades. If you were to try a typical 40 teeth blade against our 80 or 100 teeth blades you would see quite a difference in smoothness of cut whereby smoothness means a quick efficient cut without much grabbing, pulling, sticking, bogging down, or tearing (smoothness does not mean that it is not cutting, it means the opposite). The smoothness translates into less work and less effort leading to less fatigue on bigger jobs. We have professional gardener-landscapers in the Australian Outback that use only our blades as a testament.
Time saving also occurs in other ways as well (eg): Our blades don’t require wasting valuable man hours by stopping work to switch between 4 prong weed blades, then over to 40 teeth, then to 80 or 100 teeth blade types; no, our blades are the most versatile in that they cut every application which avoids wasting time switching between blade types. To support all our claims, including what we write in this response, and on our website as a whole; about no grabbing and tearing along with the durability of sharpness with our blades, we direct you to our, literally, hundreds of Customer Amazon Feedbacks and Customer Ebay Feedbacks.