Marfeel's refreshing frequency is on par with the load a single user would cause while visiting a website - there is no extra load or slowdown effect.
The biggest overload that a publisher's server would see is only during the first time Marfeel scans a server (or while actively working on it). This is caused by the thorough review the system conducts on brand new servers the first time it's exposed to them.
Once a server has been reviewed, the Marfeel platform only reviews new information placed on the server, the same way a normal user visit would.
Marfeel's crawler will always consume public facing content. If the publisher distributes its content from a CDN, Marfeel crawlers will honour it and consume the contents from it too.