diff --git a/config/build-plugins/dev-rolldown-runtime-env.ts b/config/build-plugins/dev-rolldown-runtime-env.ts
new file mode 100644
index 00000000000..af4730598be
--- /dev/null
+++ b/config/build-plugins/dev-rolldown-runtime-env.ts
@@ -0,0 +1,13 @@
+import type { Plugin } from 'vite'
+import { removeDevRolldownRuntimeEnv } from '../scripts/dev-rolldown-runtime-env.mjs'
+
+/** electron-vite starts Electron after the renderer server, so this runs once Rolldown has sized its pools. */
+export function createDevRolldownRuntimeEnvPlugin(): Plugin {
+  return {
+    name: 'orca-dev-rolldown-runtime-env',
+    apply: 'serve',
+    config() {
+      removeDevRolldownRuntimeEnv(process.env)
+    }
+  }
+}
diff --git a/config/build-plugins/dev-serve-config.ts b/config/build-plugins/dev-serve-config.ts
index 59d95dbf722..3c024b0489a 100644
--- a/config/build-plugins/dev-serve-config.ts
+++ b/config/build-plugins/dev-serve-config.ts
@@ -1,4 +1,6 @@
 import type { UserConfig } from 'electron-vite'
+import { DEV_RUNTIME_ENV_KEYS } from '../scripts/dev-rolldown-runtime-env.mjs'
+import { createDevRolldownRuntimeEnvPlugin } from './dev-rolldown-runtime-env'
 import { withBundledRendererDev } from './renderer-bundled-dev'
 
 /** Development-server overrides; Electron Vite also uses Vite's build command for the dev main process. */
@@ -8,14 +10,27 @@ export function createDevServeConfig(
 ): UserConfig {
   // run-electron-vite-dev.mjs probes a per-worktree port and reports it; strict keeps that report true.
   const rendererPort = Number.parseInt(env.ORCA_DEV_RENDERER_PORT ?? '', 10)
+  const hasPort = Number.isInteger(rendererPort)
+  // Only the dev wrapper sets runtime keys; without them there is nothing to strip.
+  const stripsRuntimeEnv = Boolean(env[DEV_RUNTIME_ENV_KEYS])
   return withBundledRendererDev(
     {
       ...config,
       renderer:
-        Number.isInteger(rendererPort) && config.renderer
+        config.renderer && (hasPort || stripsRuntimeEnv)
           ? {
               ...config.renderer,
-              server: { ...config.renderer.server, port: rendererPort, strictPort: true }
+              ...(hasPort
+                ? { server: { ...config.renderer.server, port: rendererPort, strictPort: true } }
+                : {}),
+              ...(stripsRuntimeEnv
+                ? {
+                    plugins: [
+                      ...(config.renderer.plugins ?? []),
+                      createDevRolldownRuntimeEnvPlugin()
+                    ]
+                  }
+                : {})
             }
           : config.renderer,
       main: {
diff --git a/config/scripts/dev-rolldown-runtime-env.mjs b/config/scripts/dev-rolldown-runtime-env.mjs
new file mode 100644
index 00000000000..f5771c7de32
--- /dev/null
+++ b/config/scripts/dev-rolldown-runtime-env.mjs
@@ -0,0 +1,39 @@
+import os from 'node:os'
+
+export const DEV_RUNTIME_ENV_KEYS = 'ORCA_DEV_RUNTIME_ENV_KEYS'
+
+// Measured idle electron-vite RSS 4.6GB -> 3.0GB with no HMR or startup regression.
+const DEV_THREADS = '4'
+const DEV_RUNTIME_DEFAULTS = {
+  // mimalloc otherwise keeps freed Rolldown arenas resident for the whole dev session.
+  MIMALLOC_PURGE_DELAY: '0',
+  ROLLDOWN_WORKER_THREADS: DEV_THREADS,
+  RAYON_NUM_THREADS: DEV_THREADS
+}
+
+/** Sizes the dev server's Rolldown runtime; records added keys so Electron's environment can drop them. */
+export function withDevRolldownRuntimeEnv(env, availableParallelism = os.availableParallelism()) {
+  const added = Object.entries(DEV_RUNTIME_DEFAULTS).filter(
+    ([key]) =>
+      env[key] === undefined &&
+      (key === 'MIMALLOC_PURGE_DELAY' || availableParallelism > Number(DEV_THREADS))
+  )
+  if (added.length === 0) {
+    return env
+  }
+  return {
+    ...env,
+    ...Object.fromEntries(added),
+    [DEV_RUNTIME_ENV_KEYS]: added.map(([key]) => key).join(',')
+  }
+}
+
+/** Rolldown has read these before the renderer server starts; dev Electron's terminals must not inherit them. */
+export function removeDevRolldownRuntimeEnv(env) {
+  for (const key of (env[DEV_RUNTIME_ENV_KEYS] ?? '').split(',')) {
+    if (key) {
+      delete env[key]
+    }
+  }
+  delete env[DEV_RUNTIME_ENV_KEYS]
+}
diff --git a/config/scripts/dev-rolldown-runtime-env.test.mjs b/config/scripts/dev-rolldown-runtime-env.test.mjs
new file mode 100644
index 00000000000..881e4a824cc
--- /dev/null
+++ b/config/scripts/dev-rolldown-runtime-env.test.mjs
@@ -0,0 +1,25 @@
+import { describe, expect, it } from 'vitest'
+import {
+  removeDevRolldownRuntimeEnv,
+  withDevRolldownRuntimeEnv
+} from './dev-rolldown-runtime-env.mjs'
+
+describe('dev Rolldown runtime env', () => {
+  it('removes only the keys it added, keeping caller choices', () => {
+    const env = withDevRolldownRuntimeEnv({ RAYON_NUM_THREADS: '2', PATH: '/bin' }, 24)
+    expect(env).toMatchObject({
+      MIMALLOC_PURGE_DELAY: '0',
+      ROLLDOWN_WORKER_THREADS: '4',
+      RAYON_NUM_THREADS: '2'
+    })
+    removeDevRolldownRuntimeEnv(env)
+    expect(env).toEqual({ RAYON_NUM_THREADS: '2', PATH: '/bin' })
+  })
+
+  it('keeps Rolldown thread defaults on small hosts', () => {
+    expect(withDevRolldownRuntimeEnv({}, 4)).toEqual({
+      MIMALLOC_PURGE_DELAY: '0',
+      ORCA_DEV_RUNTIME_ENV_KEYS: 'MIMALLOC_PURGE_DELAY'
+    })
+  })
+})
diff --git a/config/scripts/run-electron-vite-dev.mjs b/config/scripts/run-electron-vite-dev.mjs
index a97a879648e..a02c13b3145 100644
--- a/config/scripts/run-electron-vite-dev.mjs
+++ b/config/scripts/run-electron-vite-dev.mjs
@@ -39,6 +39,7 @@ import {
   recordDevWebClientInputs
 } from './dev-web-client-freshness.mjs'
 import { withRolldownThreadLimit } from './rolldown-thread-limit.mjs'
+import { withDevRolldownRuntimeEnv } from './dev-rolldown-runtime-env.mjs'
 import { copyPrivateTree } from './space-sharing-copy.mjs'
 import {
   DEV_BUNDLE_ID,
@@ -584,7 +585,7 @@ if (!isHelpOrVersion) {
 const forwardedArgs = ['dev', ...forwardedRaw, ...forwardedExtras]
 const child = spawn(process.execPath, [electronViteCli, ...forwardedArgs], {
   stdio: 'inherit',
-  env: process.env,
+  env: withDevRolldownRuntimeEnv(process.env),
   // Own process group so Ctrl+C terminates electron-vite’s Electron descendants on macOS too.
   detached: process.platform !== 'win32'
 })