TZ 205 slows traffic drastically

I’m running a SonicWall TZ205, SonicOS Enhanced 5.9.1.7-2o between a Spectrum cable modem and a Dell Powerconnect 2716 – though the switch does not appear the root issue.  The Spectrum cable speed is 100 Mbps.

When I connect a laptop directly to the modem and run a test using speedtest.net, I get download speeds of about 115 Mbps, and upload of 11 Mbps.  When I put the firewall between the modem and laptop and rerun the test, I get 22 Mbps download and 11 Mbps upload.  The results are the same if I connect through the switch.

While I expect some performance hit with the firewall, that seems way too much.  I’ve seen references to Content Filter Services (CFS) causing performance issues, but I tried turning it off, and that had no apparent affect, though I don’t know that I actually did it right.  Many of the search results are old and the Sonicwall interface has changed enough that it’s hard to say that I’m going down the right path.  I would certainly appreciate any suggestions to speed this up.