Paweł Kowalski

T-shaped senior software engineer with 15+ years of experience across the full stack - from pixel-perfect React frontends to distributed NestJS/Ruby/Elixir backends and AWS infrastructure. I lean toward performance problems where an optimization saves real money.

• Implementing Peer to Peer payments for 100+ million customers
• Request money feature for Klarna and non-Klarna users (NestJS) - budget: >170ms avg response time
• Allow new users to receive money before they have an account to improve user acquisition
• Send money via QR codes (Express.js, React Native) - expectation: processing 500K+ transactions weekly across 26 markets
• Phonebook synchronization in mobile apps and backend (NestJS, React Native) - expectation: 80M+/month contacts while respecting privacy laws and sub-3s sync time
• Gradual rollout of all the P2P features from 1% to 100%, from 1 market to 26 while keeping 99.9% SLA
• Keeping costs of owned services low while being able to scale to handle 50x the usual traffic (Black Friday, Holiday season)

• Technologies and frameworks used
• Developed React.js/TypeScript application inside a monorepo with hundreds of repositories - maintained ~85% test coverage
• Contributed to Ruby (on Rails) applications, large monoliths and small microservices - expectation: p95 latency >200ms
• Added Playwright Component Tests - E2E and API. Expanded RSpec coverage to reduce regressions and prevent incidents
• Used DataDog, NewRelic, Sentry, SumoLogic, Kubernetes, PagerDuty to help keep everything predictable and stable
• Development and maintenance
• Reworked licensing mechanism to accommodate enterprise customers
• Rewrote frontend for Single Sign On which allowed us to serve enterprise users - handling 200K+ monthly authentications with 2FA across SAML/OAuth providers
• Implemented CSV Team and Users import functionality across 3 micro-services - expected load: 10M+ records/month
• Improved observability by adding monitors and dashboards which resulted in fewer SEV-1 incidents in our domain

• Worked with stakeholders/designers to lower user friction and improve user experience - reduced user onboarding time by 25% while maintaining accuracy in KYC and tax document validation
• Implemented Cypress parallel test runs to reduce deployment time by 50%

• Serverless & AWS optimization
• Image processing service using S3/Lambda/Sharp - scaled down EC2 instances by offloading CPU-intensive image processing to Lambda
• Unzip service with no file size limit using S3/Lambda/Streams - scaled down EC2 instances by streaming deployment files directly to S3 and processing with Lambda, eliminating background job overhead
• Reduced AWS bill by 35% by optimizing instance types for EC2, RDS, and ElastiCache, implementing S3 storage tiers, leveraging spot instances, and tuning auto-scaling policies
• Developer advocate
• Authored articles on frontend/performance best practices to promote awareness and adoption
• Led technical development of award-winning documentation and marketing website

• Implemented capitalization table calculations using Vanilla JS - processing complex equity structures for 409A valuations

• Upgraded a legacy app to modern standards within 3 months while handling assigned tasks (load times went down from 3.4 sec to 1.9 sec on average)

• Developed and maintained post-login banking service