I'm Matt and I'm Web Performance Optimization consultant (I work only on projects related to performance optimization). I optimize both front end and backend (PHP, MySQL, low level Linux configuration). I have vast experience in PHP, MySQL optimization and I will be glad to take care of the issue for you. I have exceptionally high repeat hire rate and completion rate (actually I never failed on any project, my completion rate is 97% only because some of project owners vanished in thin air and I'm unable to finish them).
Your terms that milestones would be released only on completing the job are absolutely fine with me - I never charge for unfinished job, I never take upfront payments and I always give guarantee of success. Actually I always propose exactly the same terms to my clients - my only requirement is that you create milestones before we start.
Contrary to other freelancers I know how to optimize WordPress without usage of caching. To solve the problem I would first transfer your site to my local environment (Intel Xeon E5-2620 v3, standard dedicated server) and test it with profiler (XDebug,WinCacheGrind, kcachegrind) to identify bottlenecks. Depending on source and nature of the bottleneck(s) I taken appropriate action - optimize MySQL queries, rewrite WordPress code for better efficiency etc. (unfortunately there are way too many scenarios to consider to cover them in bid proposal).
If you think I might be good candidate for the job please contact me.
Regards,