This is a call graph from a slow test run: + ! 442 XCTAssertEqual<A>(_:_:_:file:line:) (in libXCTestSwiftSupport.dylib) + 96 [0x101727e30] + ! 442 XCTAssertEqual<A>(_:_:_:file:line:) (in libXCTestSwiftSupport.dylib) + 1308 [0x10172b5ac] + ! 442 _XCTPreformattedFailureHandler (in XCTestCore) + 148 [0x10092018c] + ! 442 _XCTFailureHandler (in XCTestCore) + 1276 [0x10091ffa4] + ! 442 -[XCTestCase(XCTIssueHandling) handleIssue:] (in XCTestCore) + 224 [0x10090d440] + ! 442 -[XCTestCase _enqueuePrimaryThreadBlock:] (in XCTestCore) + 148 [0x100946744] + ! 442 -[XCTestCase _dequeuePrimaryThreadWork] (in XCTestCore) + 340 [0x1009443fc] + ! 442 __44-[XCTestCase(XCTIssueHandling) handleIssue:]_block_invoke (in XCTestCore) + 24 [0x10090d478] + ! 442 -[XCTestCase recordIssue:] (in XCTestCore) + 948 [0x100943140] + ! 442 -[XCTestCase(XCTDeprecated) recordFailureWithDescription:inFile:atLine:expected:] (in XCTestCore) + 256 [0x10094e3cc] + ! 442 -[XCTestCase(XCTIssueHandling) _recordIssue:] (in XCTestCore) + 212 [0x10090d9a0] + ! 442 -[XCTestCase(XCTIssueHandling) _enqueueIssue:] (in XCTestCore) + 172 [0x10090da7c] + ! 442 -[XCTestCase _dequeuePrimaryThreadWork] (in XCTestCore) + 124 [0x100944324] + ! 442 -[XCTestCase(XCTIssueHandling) _dequeueIssues] (in XCTestCore) + 328 [0x10090dc7c] + ! 442 -[XCTestCase(XCTIssueHandling) _handleIssue:] (in XCTestCore) + 520 [0x10090df2c] + ! 442 -[XCTSourceCodeFrame symbolInfoWithError:] (in XCTestCore) + 148 [0x100911538] + ! 442 -[XCTInProcessSymbolicationService symbolInfoForAddressInCurrentProcess:error:] (in XCTestCore) + 588 [0x100901378] + ! 434 +[XCTSymbolicationService _symbolInfoForAddress:symbolOwner:symbolicationFunctions:allowedImageNames:error:] (in XCTestCore) + 244 [0x100900944] + ! | 434 CSSymbolOwnerGetSymbolWithAddress (in CoreSymbolicationDT) + 52 [0x100701fc4] + ! | 434 CSCppSymbolOwner::_fault_data_purging_empty(bool) (in CoreSymbolicationDT) + 48 [0x100720394] + ! | 434 CSCppSymbolOwnerCache::data_for_symbol_owner(CSCppSymbolOwner*, bool) (in CoreSymbolicationDT) + 312 [0x100721c54] + ! | 434 CSCppSymbolOwnerCache::create_symbol_owner_data(CSCppSymbolOwner*, CSCppDsymData*) (in CoreSymbolicationDT) + 428 [0x100721450] + ! | 434 create_symbol_owner_data2(CSCppSymbolOwner*, CSCppDsymData*) (in CoreSymbolicationDT) + 204 [0x10072187c] + ! | 434 CSCppSymbolOwnerData* create_symbol_owner_data_arch_specific<SizeAndEndianness<Pointer64, LittleEndian> >(CSCppSymbolOwner*, CSCppDsymData*) (in CoreSymbolicationDT) + 444 [0x1007316e8] + ! | 434 dsym_data_for_symbol_owner(CSCppSymbolOwner const*, void const*, void const*, void const*, void const*) (in CoreSymbolicationDT) + 360 [0x1006f91ec] + ! | 434 DBGCopyFullDSYMURLForUUIDWithOptions (in DebugSymbolsDT) + 80 [0x10040bef8] + ! | 434 CopyDSYMURLForUUIDWithOptions(__CFUUID const*, void const*, void const*, void const*, void const*) (in DebugSymbolsDT) + 1024 [0x10040bad4] + ! | 434 WaitForSpotlightQueryToReachState(SpotlightQueryState, int, __CFArray const*) (in DebugSymbolsDT) + 272 [0x10040a454] + ! | 434 _pthread_cond_wait (in libsystem_pthread.dylib) + 1236 [0x1a3a0883c] + ! | 434 __psynch_cvwait (in libsystem_kernel.dylib) + 8 [0x1a39ce270]

Doesn't help in my case either. Also I'd prefer to not create test plans for each package, as I have quite a lot of packages. It would be nice if it would be performant out of the box. On the upside, on my M1 Macbook Pro the delay is only 2 secs. But it's still very noticeable and gets in the way of quick change/test cycles. Edit: This was meant as a comment to the proposed solution.

FWIW Mail.app still seems to use the legacy WebView, so maybe we'll be able to enjoy the DOM API for a little longer.