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Yep, boost can be an alternative, but it needs you to modify your Drupal configuration. As we didn't develop the site (nor maintain the server), we have decided not to change anything on original server.
Besides, Cloudflare gives you great scalability - in case of boost you still have one server with limited capacity (or you have to build a cluster of servers, what means a bunch of work and maintanance). And to get best results with boost, you might need to modify your server configuration (for example - to use PHP with FastCGI) - and that's sometimes not possible (more than 1 site on server).
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