I write software for a HPC centre and we noticed this as well. The speed of our programs heavily using memory maps can vary by an order of magnitude based upon

I write software for a HPC centre and we noticed this as well. The speed of our programs heavily using memory maps can vary by an order of magnitude based upon the memory pressure on the node. This non-determinism in runtime is a show stopper for us so we ripped out most memory maps from our codebase. There is just too much magic happening under the hood to have control over your program.