This Swift program
compiled with swiftc -O test_dispatch_sync_slow.swift takes about 14s to run on my machine (with Swift 3.1 and swift-4.0-DEVELOPMENT-SNAPSHOT-2017-05-17)
This C program
however (which should be exactly the same) only takes about 2s to execute (compiled with clang -O3 test_dispatch_sync_fast.c -o test_dispatch_sync_fast).
The difference is that the Swift program spends most of the time calling Block_copy and Block_release which aren't necessary for a synchronous dispatch... See screenshot.