What version of Elysia is running?

1.4.18

What platform is your computer?

Linux 6.14.0-36-generic x86_64 x86_64

What environment are you using

Bun: 1.3.3

Are you using dynamic mode?

No

What steps can reproduce the bug?

Repo: https://github.com/HuakunShen/js-benchmark.git
README contains all the details. Run ./run-benchmark.sh to run the test.

What is the expected behavior?

I heard that Elysia is performant with bun, but the results is pretty bad with default settings. Although setting AOT = false improved performance, but the default settings is still too bad.

Is AOT enabled by default? shouldn't this improve performance? But I found performance degrade significantly with default settings.

What do you see instead?

AOT compilation causes a 45.7x performance degradation specifically on Bun runtime:

Key Issues:

• AOT causes 45.7x slower performance on Bun (3,853 vs 175,951 RPS)
• The issue is Bun-specific - AOT works fine on Deno (72,178 RPS) and Node.js (33,797 RPS)
• Default settings make Elysia practically unusable on Bun
• Performance contradicts the intended purpose of AOT compilation

Additional information

Environment

• Elysia Version: 1.4.18 (also affects 1.2 when AOT manually enabled)
• Runtime: Bun (latest)
• OS: Linux
• Benchmark Tool: wrk

Reproduction Code

import { Elysia } from 'elysia';

// This uses aot: true by default in 1.4 (broken on Bun)
const app = new Elysia()
  .get("/", "Hello Elysia")
  .get("/json", () => ({ message: "Hello World", timestamp: Date.now() }));

app.listen(3000);

Reproduction Steps

1. Create Elysia app with default settings (aot: true)
2. Run with Bun: bun run app.ts
3. Benchmark: wrk -t12 -c400 -d10s http://localhost:3000/
4. Observe ~3K RPS instead of expected ~150K+ RPS

Working Workaround

import { Elysia } from 'elysia';

// Adding aot: false fixes the issue
const app = new Elysia({ aot: false })
  .get("/", "Hello Elysia")
  .get("/json", () => ({ message: "Hello World", timestamp: Date.now() }));

app.listen(3000);

Performance Comparison with Other Frameworks

Root Cause Analysis

The issue appears to be in Elysia's AOT compiler generating code that is incompatible or inefficient with Bun's runtime. The fact that:

• Only Bun is affected (Deno/Node.js work fine)
• aot: false performs better than supposed optimization
• Performance difference is 45x (not marginal)

Suggests a fundamental incompatibility between Elysia's AOT output and Bun's execution environment.

Impact

• Severity: Critical - makes Elysia unusable on Bun with default settings
• User Impact: Silent performance regression (users expect AOT to improve performance)
• Production Risk: Default configuration cannot handle realistic traffic loads

Additional Test Results

The issue was discovered during comprehensive benchmarking across multiple load patterns:

• High load (12 threads, 400 connections): 45.7x regression
• Medium load (4 threads, 50 connections): Similar regression pattern
• Low load (1 thread, 1 connection): Regression persists

This confirms the issue is not load-dependent but fundamental to the AOT compilation process on Bun.

Have you try removing the node_modules and bun.lockb and try again yet?

Yes