90% of the bottleneck, not 90% of their whole application. The author says that rewriting everything in Rust would have taken months, so the whole application must be huge.
"It is big and complex and very business critical and highly algorithmic, so that would take ~months of work, ..."
OP fully rewrote the example program in rust, by also moving the entire data structures there. This would mean that any interaction with these ndarrays could be possible only on the rust side, hence any other code that uses them must be rewritten, unless there’s some porting of rust ndarrays to python numpy ndarrays
No? In fact, if you understand what what I wrote, by generalizing this small piece of code it would mean that most of the codebase must be also partially rewritten to rust to be able to interoperate with the new data structures moved in rust. Thus these “less than 100 lines of code” refer to just this simple example program, which was fully rewritten in rust, hence, by generalizing, the premise was pointless
"It is big and complex and very business critical and highly algorithmic, so that would take ~months of work, ..."