As usual, the folks in the orange vest have sowed half-truths and misinformation. I don't doubt that the saws that your local store is renting are underpowered and not suited to cutting granite, but not for the reasons you were given. Speed is a cutting issue, but any professional quality saw will have enough speed and they would have no business renting equipment that wasn't of a professional grade.
What's more important is the quality of the blade on the saw and whether or not the saw is cutting square or needs adjustment. A professional rental yard will have a blade specific to cutting granite tile and will likely charge you for the wear you put on the blade, which they check with a micrometer. A professional rental yard will also likely have maintained their equipment properly and they will be able to confirm that the saw they rent to you will cut squarely.
Best of luck,
Capua Custom Tile & Stone; Seattle, Washington
Member- National Tile Contractors Association
Card-carrying Schluter Systems installer
Certified WEDI Board installer
NuHeat radiant heating system "Qualified Installer"
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