Clean up & Optimise our current eCommerce Store

We have had our online store since 2012 and it has become slow and the theme slightly dated. We would like to engage the services of a web developer with shopify specific experience, not just using templates but custom coding in CSS within the liquid templates to produce custom elements. We need that person to go through all of our liquid files and remove redundant code, identify what is slowing the website down and help us to optimise it to its maximum potential, we are starting to receive complaints from customers about the very slow speed of our website. The current theme we have has been customised so many times and had so many applications loaded and unloaded that we have much useless code in our liquid files we are not experts but we think that this might be slowing the whole site down. We have tried optimising images etc..but nothing is working. The company that does this project must have proven shopify specific development experience and high star rating and reputation within this exact field. We look forward to working with the most successful bidder. Thanks