Entirely reasonable doubt, you can find well-commented benchmark output of the Laser High-Performance Computing lib and Arraymancer lib in their source:

https://github.com/numforge/laser/blob/d1e6ae6106564bfb350d4...

https://github.com/mratsim/Arraymancer/blob/6fddfa9a734ac01c...

To qualify the statement, it would be more accurate to say that Nim can smoke C by several orders of magnitude unless you devote significant engineering efforts into replicating the equivalent Nim (as Nim targets C/C++ regardless, so this is just heavy lifting and optimizations)