This may be generally true (though the benchmarks I've seen show Knet.jl and sometimes Flux.jl on par with TF/PyTorch with a single machine + single GPU), but there are definitely domains where it is categorically not. The most prominent one is neural *DEs, where the SciML [1] ecosystem has SOTA performance. You can really see Python/C++-based frameworks struggle here because they have slow "glue code" and don't (one could argue can't effectively) optimize for latency. That's not a problem for most CV models and transformers, but really stunts research into more dynamic approaches.

[1] https://sciml.ai/