This is an old 2017 answer of mine from Quora that has been deleted by moderators for violating their rules, kept here for historical interest:
Many people here are claiming "yes" that Swift is fast. However, all of the benchmark results I can find indicate that Swift is much slower than most other languages, up to 24x slower than C++:
- n-body: swift 24.09s vs Java 22.6s.
- fannuck-redux: swift 59.5s vs Java 17.41s. Swift is 3.4x slower than Java.
- spectral-norm: swift 15.7s vs Java 4.28s. Swift is 3.7x slower than Java.
EDIT: The shootout appears to have been updated. The new Swift code is still memory unsafe and still compiled in memory unsafe -Ounchecked mode. Swift now beats Java on 3/7 tasks (fannkuch-redux, mandelbrot and binary-trees), roughly draws on two (nbody and fasta-redux) and loses on two (fasta and spectral norm). Shame there is no memory safe Swift for comparison. I’d also note that many of these benchmarks are apples vs oranges comparisons with, for example, the Java and Swift implementations of binary-trees using completely different algorithms and data structures.
Other (non-Java) benchmarks found:
- Swift running 11x slower than C because "The dynamic memory allocation turned out to be a major performance killer" in
- "Swift is 4x slower than Objective C"
- "Swift is 24x slower than C++"
- Swift serializing JSON 15x slower than Objective C in
- Memory-safe Swift is running 22x slower than C in .
- Swift ran up to 10,000x slower than C++ in although memory-safe Swift is now (Xcode 6 beta 5) running as fast as C there with ordinary optimisations enabled.
My impression is that Swift was vastly slower than almost all other languages at many common tasks when it was first released but it has improved very rapidly. However, it still seems to be several times slower than C++ in many cases and, therefore, I expect it is still significantly slower than languages like F# and Scala.
I am also concerned about many of the benchmarks being used. The shootout is notoriously bad science with submissions subjectively "de-optimised" by its owner for being too fast rendering the results total garbage. Numerical benchmarks like DGEMM and FFTs are largely irrelevant. I am much more interested in symbolic performance, not just because that is more relevant to the code I write but because it will stress Swift's reference counting garbage collection which I suspect will be its Achilles heel. The JSON serialization benchmark was very interesting to me as a consequence. I am also disturbed by the use of optimisations that remove memory safety in Swift (-Ounsafe) and the use of inaccurate numerics in C++ (-ffast-math).
A post on Hacker News () says that short-lived objects in the JSON serializer are to blame for the poor performance. In other words, the reference counted memory management is the problem as I suspected.