If I understand the SiteGround’s answer correctly, what they turned off is SSD-based cache for nginx, so now for most of the resource requests the nginx server will have to fetch the resource from the Apache server, instead than trying to first locate it in the SSD cache.
I don’t think it should affect the scene download time much, since the user’s internet connection will still be the bottleneck for a Shapespark scene.
BTW, not sure if you’ve seen this quite recent feature: Custom domain support If having your own domain in the scene URL is the most important reason for own WWW hosting, then the alternative could be using this feature, which allows to use our hosting infrastructure (and upload scenes with the “Upload” button) with customer’s own domain.