Using CFS, Ping is fine, but the Sonicwall web interface shows high CPU usage and large downloads break or get extremely slow.
In most cases, the Sonicwall Content Filtering Service (CFS) causes this issue.
You might disable it temporarily to see a difference: Network -> Zones -> Disable content filter service e.g. for LAN
Also try Security Services -> Summary -> Optimize for performance.
For me, the best workaround was to completely disable the CFS and use a free alternative instead. A free DNS filter like Nxfilter seems to be a good solution.