There are some legitimate criticisms to be made of Swift, particularly in regards to tooling. But if you feel obliged to downvote somebody for making a factual, if anecdotal, claim about their experience using it, you might want to question how much of your reaction is just resistance to change.

How do you do DSP work in Swift?

The same way you'd do in Ruby: call an optimized C library.

My synth app is still a C++/ObjC app. Integrating with C++ code is one area where ObjC still wins.

vDSP is exposed to Swift: https://developer.apple.com/documentation/accelerate/vdsp