Your problem just exactly the same what my team have to solve 2 years ago because the customer require too much from the performance. But still I am pretty sure they will accept the hosting quality of
SiteGround Magento Hosting package with package name GrowBig include supercacher to speed up magento. And of course you need to choose the server location in US.
SiteGround have all of things you need to a top magento hosting in US and caching, CDN, US native support, phone call support, free moving, magento experience support, magento server compatible, memcached, auto update, security ... and much more. Almost what you are looking for to have best performance for magento they already got there with very fast support.
Let 's try and see if it work for you. Hope it help.
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