Principal JavaScript Performance Engineer (with C++ / Java / C# exposure) - London

Principal JavaScript Performance Engineer (with C++ / Java / C# exposure)

Location: London (Hybrid – 2 days WFH)

Salary: £150,000 - £200,000 (Permanent)pension, private healthcare, gym membership, 25 days + 8 BH holiday allowance, tech/hardware budget.

A high-growth software company is building the gold standard in UI frameworks and data-intensive visualisation technology . With millions of weekly downloads and adoption by the world’s largest financial institutions and technology businesses, their product powers mission-critical applications across the Fortune 500.

They are now hiring exceptional engineers who want to work at the edge of what’s possible in the browser — solving problems of scale, speed, and complexity that ordinary web development never touches.

What you’ll do

• Design and optimise a high-performance UI framework used globally in real-time applications.
• Push the limits of JavaScript, TypeScript, and the browser runtime to handle millions of data points smoothly.
• Solve deep algorithmic and architectural challenges in rendering, scalability, and memory efficiency.
• Own features end-to-end: design, code, test, release, and iterate.
• Contribute to a product used by developers and enterprises worldwide.

What you’ll bring

• Mastery of vanilla JavaScript and TypeScript , with additional exposure to C++, Java, or C# .
• Strong knowledge of data structures, algorithms, and performance optimisation .
• Experience with frameworks like React, Angular, or Vue , and a deep understanding of DOM internals and rendering pipelines .
• A systems-level mindset : diagnosing bottlenecks, eliminating memory leaks, optimising rendering and throughput.
• Passion for building scalable, developer-facing libraries and frameworks , not just apps.

Why join?

• Shape the future of UI frameworks at massive scale , tackling problems most engineers never encounter.
• Competitive salary £150,000 – £200,000 plus bonus and benefits.
• Engineering-first culture: flat structure, no bureaucracy, high-calibre peers.
• Hybrid work model: central London office, two days WFH.
• Premium benefits: private healthcare, pension, gym membership, cutting-edge hardware.

If you’re ready to engineer the engines behind some of the world’s most demanding applications, we’d love to hear from you.

Apply now and build technology trusted by the Fortune 500.