My webhost uses Varnish Cache, so, when I upload a revised webpage by ftp using the same filename and the Replace option, the changes are not reflected in the displayed webpage for some time. That's annoying for me, and I've been told how to turn off Varnish Cache, but I'm concerned that turning it off would degrade the website's performance in visitor browsers. Are my concerns well founded?
Well, the main purpose of this product, Varnish Cache, is to provide web acceleration. You can think of this as a reverse proxy service. When HTTP requests come into the hosting infrastructure, they pass through the varnish cache first. This infrastructure will attempt to respond to the requests using information stored in the cache instead of passing the request to the back-end system(s). How will this affect your site's performance? Hard to say because it depends on quantity of cachable content from your site. The more that this system caches, the better the performance.