I provide time and again Magento consultancy especially pertaining to performance optimization to GtMetrix. I have optimized Magento sites for several clients.
Rewritten Magento Indexers to use the indexes optimal and implemented table rotation. Also optimized the queries.
I wrote and executed a full load test suite for a high-traffic Magento store. We successfully emulated up to 5 orders per second, caught major performance bugs from release to release, and fine-tuned MySQL in AWS's RDS for peak write throughput.