Retrospective: Adopting Svelte 5 for Our Component Library – 30% Smaller Bundle Size and Faster Development

At our company, we maintain a shared component library used across 8+ frontend products, powering everything from internal dashboards to customer-facing web apps. For the past two years, the library has been built with Svelte 4, and while it served us well, we started hitting pain points as the library grew to 120+ components: bundle bloat, verbose reactivity boilerplate, and slower development cycles for new components.

When Svelte 5 launched with its reworked reactivity system (runes), smaller runtime, and improved compiler optimizations, we knew it was time to evaluate a migration. Six weeks later, we’ve fully migrated the library to Svelte 5, and the results speak for themselves: a 30% reduction in minified and gzipped bundle size, and a measurable boost in development velocity.

Why We Evaluated Svelte 5

Svelte 4’s reactivity relied on assignment-based detection, which worked well for simple cases but led to confusing edge cases as our components grew more complex. We also found ourselves writing more boilerplate to manage derived state and side effects, and the Svelte 4 runtime added unnecessary weight to our bundle as we added more components.

Svelte 5 addressed these pain points head-on. Its new runes system ($state, $derived, $effect) makes reactivity explicit and predictable, eliminating the guesswork around when updates trigger. The Svelte 5 compiler also produces more optimized output, with better tree-shaking and a 60% smaller runtime than Svelte 4. For a shared component library where every kilobyte counts, this was a game-changer.

We also valued that Svelte 5 maintains backwards compatibility with Svelte 4 components, allowing us to migrate incrementally instead of rewriting the entire library at once.

The Migration Process

We split the migration into three phases to minimize disruption:

1. Phase 1: Low-risk components – We started with simple, stateless components (buttons, badges, icons) that had no complex reactivity. This let our team get familiar with runes without risking breaking changes to critical features.
2. Phase 2: Stateful components – Next, we migrated components with internal state (form inputs, modals, dropdowns) using the official Svelte 5 migration tool to automate most of the conversion. We manually reviewed each converted component to fix edge cases, like legacy $: reactive statements.
3. Phase 3: Critical components – Finally, we migrated high-traffic components (data tables, navigation bars, layout grids) with extra testing to ensure no regressions. We also updated our library documentation and added runes-specific examples for the team.

The entire migration took 6 weeks with a team of 3 frontend engineers, with no downtime for products using the library.

Results

30% Smaller Bundle Size

Our library’s minified and gzipped bundle size dropped from 42KB to 29KB – a 30% reduction. We attribute this to three Svelte 5 improvements:

• A 60% smaller runtime: Svelte 5’s runtime is stripped down to only the core functionality needed for runes-based reactivity.
• Better tree-shaking: The Svelte 5 compiler eliminates more dead code, so unused component logic isn’t included in the final bundle.
• Smaller component output: Runes produce less boilerplate code than Svelte 4’s assignment-based reactivity, reducing the size of each compiled component.

For our products that use the full library, this translates to faster initial load times, especially for users on slow networks.

Faster Development Cycles

We’ve seen a 25% reduction in time spent building new components, thanks to runes eliminating reactivity-related boilerplate. Developers no longer have to debug confusing assignment-based reactivity issues, and the explicit nature of runes makes component logic easier to read and maintain.

Onboarding new team members has also gotten faster: runes are more intuitive for developers familiar with other reactive frameworks, and the Svelte 5 IDE extension provides better autocomplete and error checking for runes-based code.

Lessons Learned

No migration is without hiccups. Here are the key lessons we picked up along the way:

• Incremental migration is non-negotiable – Trying to rewrite everything at once would have caused weeks of downtime. Svelte 5’s backwards compatibility made incremental migration seamless.
• Runes have a learning curve – Developers used to Svelte 4’s $: reactive statements took ~1 week to get fully comfortable with runes. Pair programming and internal workshops helped speed this up.
• Test edge cases thoroughly – We found a few edge cases with legacy Svelte 4 patterns (like mutating arrays without assignment) that broke when migrated. Adding snapshot tests for all components caught these early.
• Update documentation early – We made the mistake of migrating components before updating our internal docs, leading to confusion for teams using the library. Updating docs alongside migration would have saved time.

Conclusion

Adopting Svelte 5 for our component library has been a resounding success. The 30% bundle size reduction and faster development cycles have already delivered tangible value to our team and our users. For any team using Svelte 4 for a component library or large application, we highly recommend evaluating Svelte 5 – the migration effort is well worth the results.

Next, we’re planning to adopt Svelte 5’s new snippets feature to reduce duplication in our component templates, and we’re exploring SvelteKit to rebuild our library’s documentation site.