Slow cutting usually occurs when a diamond blade is used to cut a material that is too hard for the segment being used. The hard material wears the exposed diamonds, but the metal matrix holding the diamond grit in place due to its hardness, does not wear proportionately.
The segment then becomes glazed over, overheats, bounces in the material being cut and finally fails to cut, leading to cracks in the core or segment loss.
When this occurs and is noticed before the blade loses its tension, the best solution is to run the diamond blade in a soft abrasive material like a masonry block until the diamonds in the matrix are freshly exposed.
A diamond blade professional should be consulted to help with specifying the correct blade for the material to be cut.
Check the flanges are fitting correctly and that the rpm rating is correct. Always follow the guidelines for proper blade operation and cooling procedures.