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Lovable rebuilt Vite's dev server in Rust to pack more sandboxes onto each host
OJ moves the file watcher, module graph and hot updates into a Rust binary and starts Node only for plugins. Lovable reports median sandbox acquisition dropping from 14.5 seconds to 3 seconds, and Vite's creator calls the headline cold-start benchmark somewhat misleading.
The Engineer · Build desk
What happened
- Lovable has started replacing Vite inside its cloud preview environments with OJ, a Rust build tool written for running around one million short-lived development sandboxes a day.
- In a controlled production rollout the company reported median sandbox acquisition time falling from 14.5 seconds to 3 seconds, and dev-server process memory dropping by roughly 6.5 times.
- Both OJ and Vite bundle with Rolldown and parse with Oxc, the VoidZero projects that already power Vite 8.
- Against Vite's own bundled development mode, the current benchmark puts cold start at 1,315 milliseconds for OJ and 1,528 milliseconds for Vite on a 10,000-component test.
- In that same test OJ used 115 MB of memory against 1,751 MB for Vite's bundled mode, while hot module replacement was effectively even between them.
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Why it matters
- cost Memory cannot be oversubscribed the way CPU can, so the dev server's idle footprint, not its cold start, is what decides how many previews Lovable fits on a host.
- constraint Anyone reusing these figures inherits Lovable's plugin set: the TypeScript checking OJ skipped still has to run somewhere, and running it brings the Node host back.
- precedent If rebuilding one layer of a dependency to fit house infrastructure becomes routine, maintainers get benchmarked against forks that only have to support a subset of their paths.
- contradiction Which Vite mode you pick as the baseline changes the speed verdict from fourfold to a sixth, and the memory gap holds across that choice while the speed gap largely does not.
The Rust binary owns the dev server. It reads an existing `vite.config.ts` and runs Vite and Rollup-style plugins through a compatibility bridge, starting a small Node process when an application needs JavaScript plugins or server-side modules [4]. "It is not an entire rewrite of Vite," Vite creator Evan You said on X [5]. The Node process comes back whenever a config asks for it. Plugin-free, OJ's memory advantage in the 10,000-component test is about fifteen times [1]. With the Node plugin host running against a real `vite.config`, OJ creator Raphael Amorim said tests still showed OJ using less than one-third as much memory [13].
The rollout ratio Lovable reported is about 43 percent of the plugin-free lab ratio [6]. Its production figure sits between those two. Lovable engineer Jonathan Grahl explained why the company optimises for that axis: "Memory is a dedicated resource (compared to CPU which is highly shareable); which makes it expensive," he said [11]. He said the dev server was idling at around 400 MB, and that Lovable hoped to push it lower so it could pack more sandboxes onto each host [12]. That idle number is roughly 3.5 times what OJ used in the benchmark [2].
Lovable first put bundle-mode OJ against Vite's default unbundled development mode: about 1.2 seconds to cold start versus 4.9 [7], roughly four times [5]. You called that benchmark "somewhat misleading" because Vite has its own bundled development mode [8]. Bundled against bundled, the gap is 213 milliseconds, about 16 percent [3]. Lovable also disclosed that one of its real-application comparisons ran `vite-plugin-checker` on the Vite side, which starts a TypeScript worker that OJ skips because it does not support the plugin [14].
Lovable said the production gains came partly from a smaller binary and sandbox image cutting provisioning time before an app could serve [15]. The 14.5 seconds covers sandbox acquisition, not dev-server start. The saving is 11.5 seconds per acquisition, a 4.8x reduction [4], and it holds only if you are pulling a fresh sandbox image per session around a million times a day [1] and counting that pull inside your dev-server metric. On a laptop that starts one dev server after breakfast, what you get is the 213 milliseconds [3].
pnpm's Rust rewrite was led by its original maintainers, Rspack preserves much of webpack's configuration and API, and Biome ships a Rust formatter with high Prettier compatibility [18]. socket.dev distinguishes OJ from those by its target: one layer of a general-purpose dependency, rebuilt around a single company's infrastructure, in a piece headlined around AI lowering the cost of forking open source [19][23]. The support offered for the AI part is You's expectation that the approach will become much more common [19].
Vite covers a wide range of frameworks, plugins, application patterns and downstream tools; OJ only has to cover the paths Lovable's preview sandboxes use [21]. That narrower coverage is what produces the numbers. The compatibility work Amorim documented is the cost of staying close to Vite: regex aliases, source files outside the application root, TypeScript enums, `import.meta.env`, plugin virtual modules [17]. "These are not toy apps," he wrote of Excalidraw and Twenty, which "both run on OJ unchanged" [16]. Twenty's frontend alone contains roughly 15,000 modules [17].
What to watch
- Whether Lovable gets the dev server below its 400 MB idle figure, which is what sets how many sandboxes fit on a host.
- Whether OJ adds vite-plugin-checker support, letting the largest speed comparison be rerun with the TypeScript worker running on both sides.
- Whether other companies publish single-layer forks of general-purpose tooling and benchmark them against the general tool.