A decade ago, I worked on a simulation program that involved a C++ core with a Python wrapper and DB interface. End users cared a lot about throughout with a rather limited size, weight and power budget. We spent a lot of time optimizing the core -- but basically hit a bottleneck once we got to about 15% of the time that was spent in malloc-related functions. It turned out that was all in the Python layer. Probably there was some level of bad code in our Python code, but it was impractical to figure out where that was. I was shocked because I assumed the simulation core (which ended up almost allocation-free by the end) would always dominate CPU usage.

Both allocators and Python have probably gotten better since then, but it was a fascinatingly large and stubborn fraction of CPU time.