Performance and Best Practices

When to use this reference

Use this file when test runs are slow, flaky, or not scaling in CI, and when you need practical patterns for fast, deterministic Swift Testing suites.

Core principles

• Prefer deterministic tests over timing-sensitive tests.
• Prefer synchronous verification when asynchronous waiting is not required.
• Keep tests independent so parallel execution remains safe and effective.
• Treat .serialized as a temporary compromise, not a default architecture.

1) Keep tests synchronous where possible

Synchronous tests generally run faster and are easier to reason about.

import Testing

struct PriceCalculator {
 static func total(_ subtotal: Int, discount: Int) -> Int { subtotal - discount }
}

@Test func totalCalculation() {
 #expect(PriceCalculator.total(100, discount: 20) == 80)
}

Avoid introducing async or sleeps for purely synchronous logic.

// Avoid:
// @Test func totalCalculation() async {
//   try await Task.sleep(nanoseconds: 100_000_000)
//   #expect(...)
// }

2) Avoid unnecessary main-actor isolation

@MainActor can reduce useful parallelization and should be used only when code truly needs main-thread isolation.

import Testing

// Good: non-UI logic stays non-main-actor.
@Test func parserIsStable() {
 #expect("A,B,C".split(separator: ",").count == 3)
}

// Use @MainActor only for UI/main-thread sensitive code.
@MainActor
@Test func viewModelMutation() {
 #expect(true)
}

3) Remove shared mutable state

Shared mutable state is a major source of flakiness and parallel failures.

import Testing

enum Globals {
 static var token: String?
}

// Flaky pattern:
@Test func writeToken() {
 Globals.token = "abc"
 #expect(Globals.token == "abc")
}

@Test func expectsNoToken() {
 #expect(Globals.token == nil)
}

Better: create isolated state per test.

import Testing

struct SessionState {
 var token: String?
}

@Test func isolatedTokenState() {
 var state = SessionState()
 state.token = "abc"
 #expect(state.token == "abc")
}

4) Prefer in-memory dependencies for the fast path

Use fakes/in-memory repositories for high-volume test runs; reserve real integration dependencies for dedicated plans.

import Testing

protocol CacheStore {
 func put(key: String, value: String)
 func get(key: String) -> String?
}

final class InMemoryCacheStore: CacheStore {
 private var values: [String: String] = [:]
 func put(key: String, value: String) { values[key] = value }
 func get(key: String) -> String? { values[key] }
}

@Test func cacheRoundTrip() {
 let cache = InMemoryCacheStore()
 cache.put(key: "user", value: "42")
 #expect(cache.get(key: "user") == "42")
}

5) Use parameterized tests to reduce overhead and improve diagnostics

import Testing

func isValidPort(_ value: Int) -> Bool { (1...65535).contains(value) }

@Test(arguments: [1, 80, 443, 65535])
func validPorts(_ port: Int) {
 #expect(isValidPort(port))
}

This reduces duplicated setup code and gives argument-level failure visibility.

6) Keep setup cheap and scoped

• Build expensive fixtures only when needed.
• Prefer per-suite immutable setup for shared readonly data.
• Avoid network/file-system setup in unit tests unless behavior depends on it.

import Testing

struct CurrencyTests {
 let rates: [String: Double]

 init() {
 rates = ["USD": 1.0, "EUR": 0.92]
 }

 @Test func hasEURRate() {
 #expect(rates["EUR"] != nil)
 }
}

7) Use .serialized narrowly

import Testing

@Suite(.serialized)
struct TemporarySerialDBTests {
 @Test func migrationA() async throws { #expect(true) }
 @Test func migrationB() async throws { #expect(true) }
}

Add TODO context and remove once dependencies are isolated.

8) Flakiness reduction checklist

• No reliance on execution order.
• No shared mutable globals/singletons without reset.
• No arbitrary sleeps as synchronization.
• No hidden external dependencies in unit tests.
• Deterministic fixtures and stable clocks/random sources.
• Explicit known-issue wrappers for temporary failures.

Quick do / don't

• Do optimize for determinism first, then speed.
• Do keep most tests parallel-safe and dependency-light.
• Don't treat test slowness as only a hardware problem.
• Don't move everything to @MainActor or .serialized to silence flakiness.