How Can I Reduce Build Times in Large Monorepos?

Introduction

Large monorepos store many projects, services, and libraries in a single repository. While this setup improves code sharing and consistency, it often leads to slow builds. As teams grow and codebases expand, build times can increase from minutes to hours, slowing down development and CI/CD pipelines.

In simple terms, long build times usually happen because too much work is being done per change. The solution is not one single trick, but a set of practical optimizations that reduce unnecessary work. This article explains proven ways developers can reduce build times in large monorepos, using plain language and real-world examples.

Build Only What Changed (Incremental Builds)

One of the biggest causes of slow builds is rebuilding everything for small changes. Incremental builds focus only on the parts of the codebase that actually changed.

Basic idea:

Small change → Build only affected projects

Instead of rebuilding all apps and libraries, tools track dependencies and rebuild only the affected components.

Use Dependency Graphs to Understand Impact

Monorepos work best when dependencies are clearly defined. A dependency graph helps the build system know which projects depend on which libraries.

Example concept:

App A → Shared Lib X
App B → Shared Lib Y

If Lib X changes, only App A rebuilds. App B stays untouched. Clear dependency boundaries save massive build time.

Enable Build Caching

Build caching avoids repeating work that was already done. If the inputs are the same, the output can be reused.

Example flow:

Same input → Cached output → Skip rebuild

Caching can be local (developer machine) or remote (shared across CI and team machines).

Use Remote Cache for CI and Teams

Remote caching allows multiple developers and CI pipelines to share build results.

Example idea:

Developer A builds → Output cached remotely → CI reuses it

This dramatically reduces CI build times, especially for large teams.

Split Builds into Smaller Independent Tasks

Large builds often fail because everything is bundled into a single step. Splitting builds into smaller tasks improves parallelism and reliability.

Example structure:

Lint → Test → Build → Package

Each step can run independently or in parallel when possible.

Run Tasks in Parallel

Most modern CI systems and build tools support parallel execution.

Parallel idea:

Service A build  ┐
Service B build  ├→ Run at same time
Service C build  ┘

Parallelism reduces wall-clock time even if total work stays the same.

Avoid Full Clean Builds by Default

Clean builds remove all previous outputs, forcing a full rebuild. While useful occasionally, clean builds should not be the default.

Better approach:

Normal build → Reuse outputs
Clean build → Only when required

This saves time during day-to-day development.

Reduce Unnecessary File Watching

Large monorepos often watch thousands of files. Excessive file watching slows down builds and developer machines.

Example improvement:

Watch only active projects → Ignore generated files

This reduces CPU and memory overhead.

Optimize Test Execution

Tests often take more time than builds.

Common optimizations:

Run only affected tests
Parallelize test suites
Separate unit tests and integration tests

Example idea:

Change in UI → Run UI tests only

Avoid running the entire test suite for every small change.

Use Prebuilt Artifacts for Shared Libraries

Shared libraries that change rarely should not be rebuilt every time.

Example approach:

Stable library → Prebuilt artifact → Reuse across projects

This reduces repeated compilation work.

Optimize CI Pipelines for Monorepos

CI pipelines must be monorepo-aware. A naive pipeline rebuilds everything on every commit.

Better pipeline logic:

Detect changed paths → Trigger relevant jobs only

This can reduce CI time by a large margin.

Avoid Over-Abstracting Shared Code

Too much shared code increases dependency chains. A small change can trigger rebuilds across the entire repo.

Better practice:

Shared only when needed → Localize changes

Balanced modularity improves build performance.

Monitor and Measure Build Performance

You cannot optimize what you cannot measure. Teams regularly track build duration, cache hit rates, and slow steps.

Example metrics:

Build time
Cache hit percentage
Slowest tasks

Data-driven optimization produces long-term results.

Keep Tooling and Infrastructure Updated

Outdated build tools and CI runners are often slower.

Example maintenance:

Upgrade build tools → Faster execution
Upgrade runners → Better parallelism

Small upgrades can result in noticeable improvements.

Summary

Reducing build times in large monorepos requires focusing on incremental builds, dependency awareness, caching, parallel execution, and smarter CI pipelines. By building only what changes, reusing cached outputs, optimizing tests, and avoiding unnecessary work, teams can dramatically reduce build times without sacrificing reliability. Over time, these practices lead to faster feedback, happier developers, and more scalable monorepo workflows.