After enabling CFS, we see higher bandwidth consumption / slower speeds. What might be the cause and how can we mitigate it?
Enabling CFS typically opens up more bandwidth than it consumes, as it drops unwanted connections. Only queries to the CFS cloud servers are added for new domain lookups. However, there are reasons that you may be experiencing something that SEEMS like additional bandwidth, such as CFS host tag enforcement when a site does not have a host tag, as discussed in the following article: Unable to access certain websites, either slow or completely failing.
It is also advisable to upgrade to check the version of firmware you are on, as some previous early release versions of firmware were known to impact throughput when using CFS, and prevented them from moving to general release. This issue that was resolved some time ago, but the latest firmware must be applied for this to take effect.