Fast by principle, lean by default. Sustainable web performance is not optional: it makes services useful, inclusive, and economical. We start with a technical and usage audit: real metrics, Core Web Vitals, traces, performance budgets, assets, scripts, journeys. We identify bottlenecks (rendering, requests, images, bundles), define budgets, and implement measurable optimisations. Tool-side, we rely on Lighthouse and Ecograder to track progress and engage teams. Performance only matters if it serves usage. We collaborate with UX Design to simplify journeys, reduce cognitive load, and rationalise the Design System (lean components, shared states). We improve accessibility (semantic structure, contrast, focus, media alternatives), because accessible products are naturally faster. Tech-wise, we adjust APIs and API automation (caching, pagination, back-pressure, webhooks), strengthen CI/CD, and enable clear monitoring. If AI integration makes sense (semantic search, internal assistants), we frame it to avoid cost or latency spikes. To anchor change, we propose a two-phase plan: visible quick wins in a few days (compression, lazy-loading, asset strategy, budgets), then sustainable projects (front/back refactoring, data architecture, caching policies, media governance). We connect everything to consulting: strategic framing to align effort, product governance to maintain quality over time, and training-sensitisation to spread digital sobriety and responsible tech practices. Deliverables: performance report, budgets, dashboard, sequenced optimisation plan, review checklist, Design System conventions for lean design, accessibility recommendations. Outcome: faster, better indexed, more satisfying services, with controlled infrastructure costs — and active bridges to UX Design, audits, and product governance.