I need an expert in Wordpress performance tuning and multisite configuration

General information for the website: Wordpress multisite with 10 sites on it which need speeding up
Description of every page/module: We have set up a wordpress multisite but need a performance tuning expert to help us speed it up. Need a developer with experience configuring multisites to check we have it set up in the best way, and experience enhancing the performance of wordpress websites. One website seems to be being slowed down because of the style changes we have made in the child theme we have created. Need to know about caching and be able to work with Windows and Linux servers as we want to rebuild the multisite on Linux to improve performance.
Description of requirements/features: speed, performance, set up
Extra services needed: Customization, Optimization
Extra notes: main website we are having issues with is hickmet.com others on the network are daylight-signs.com nightpastures.com ukngroup.worxhub.co.uk