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Sergii Kryvenko

@sergiikryvenko

Senior Frontend Engineer at Opinion Stage who rebuilt the product interface with a design system and raised Lighthouse scores to 90+.

Ukraine

Message

At Opinion Stage, I led the migration of the widget editor to a new React architecture and created a design system for the product interface. I improved the Lighthouse performance score from 67–75 to 90+ and brought the embeddable widget to WCAG 2.1 AA compliance.

At Google | Google Stadia, I built performance-critical React interfaces for a cloud gaming platform streaming games at 4K/60fps. At Syndicode | Fuzu, I developed the React and MobX UI for a career platform with 100k+ monthly active users, reducing initial load times for mobile users on 2G/3G by 30%+.

Across my frontend work, I’ve focused on design systems, predictable data flow and web performance. Recently, I integrated OpenAI GPT and Google Gemini into content workflows, cutting manual processing time by 60%.

Experience

Work history, roles, and key accomplishments

Current

Senior Frontend Engineer / Lead

Current

Opinion Stage

Apr 2025 - Present (1 year 6 months)

Led migration of the widget editor to a new React architecture, improved Core Web Vitals, and achieved WCAG 2.1 AA compliance. Integrated OpenAI GPT and Google Gemini into content workflows, cutting manual processing time by 60%.

React TypeScript Core Web Vitals WCAG 2.1 AA Design Systems OpenAI GPTs Gemini Storybook Product Analytics

Senior Frontend Engineer / Tech Lead

Freelance | Upwork

Jan 2023 - Mar 2025 (2 years 2 months)

Led React and TypeScript frontend work across 5+ client engagements, shipping AI SaaS platforms, warehouse management systems, and data visualization tools. Improved LCP from 10 seconds to under 2 seconds and integrated AI features.

React TypeScript AI OpenAI GPTs Gemini Performance Optimization Architecture Stakeholder Communication

Senior Frontend Engineer / Lead

Syndicode | Evrlearn

Dec 2021 - Jan 2024 (2 years 1 month)

Led React and TypeScript frontend development for a personalized learning discovery platform, designing search, filtering, and recommendation systems. Built data layer with TanStack Query, improving perceived load times by up to 40%.

React TypeScript Tanstack Query SEO Core Web Vitals Prerender.io Caching Optimistic Updates

Senior Frontend Engineer / Lead

ClearCryptos

Apr 2022 - Jan 2023 (9 months)

Led React frontend for a cross-chain DEX with real-time swap aggregation across 5+ blockchain protocols. Shipped multi-chain portfolio analytics and a fiat on-ramp, and unified UI across React and Blazor parts.

React Blockchain Dex Portfolio Analytics UI Consolidation Blazor

Frontend Engineer

Syndicode | Fuzu

Apr 2020 - Oct 2022 (2 years 6 months)

Developed React and MobX UI for Africa's leading career platform with 100k+ monthly active users. Reduced initial load times by 30%+ for mobile users on 2G/3G networks using code splitting and SSR caching.

React MobX Code Splitting SSR Caching Canvas APIs React Spring Performance Optimization

Frontend Engineer

Syndicode | MedYouCate

Nov 2021 - Mar 2022 (4 months)

Introduced SSR with Next.js on a medical education platform, raising Lighthouse performance score from 50 to 93-95. Created interactive medical case studies and refactored the platform to meet WCAG 2.1 AA.

Next.js SSR React WCAG 2.1 AA Performance Optimization Lighthouse

Frontend Engineer (Contract)

Google

Jun 2019 - Jan 2020 (7 months)

Built performance-critical React interfaces for Google Stadia, a cloud gaming platform streaming games at 4K/60fps. Tuned React rendering and applied code splitting to reduce re-renders and initial bundle size.

React Performance Optimization Code Splitting Cloud Gaming