SSLVPN way too slow!

I'm supporting SonicWall for our customers since 2002, so I know the basics ;-)

I tried everything I could find in the knowlegebase and the forums, I have no idea why SSLVPN is so slow. I opened a ticket with SonicWall support (#44061153) and the support technican told me that I've configured everything correctly and that SSLVPN simply isn't faster! I can't believe I only get 64 MBit/s on a 1000 MBit/s connection!

To avoid the "SMB over WAN connections" problems, I did all my testing with FTP. The customer has a symetrical 1000 MBit/s fibre connection and a TZ500. At my end I have a 1000/50 MBit/s connection. I did all my testing on a Mac with "Mobile Connect" and on a PC with NetExtender. Everything (firmware, operating systems, client software) on both ends is up to date.

First I opened the FTP ports WAN->LAN for a few minutes and downloaded a huge files from the customer to my computers. I got a download speed of about 170 MBit/s. Not really fast on a 1000 MBit/s connection, but this is what the connection is able to.

Then I closed the WAN->LAN ports again, connected with the SSLVPN client and downloaded the same file over the same connection to the same computers: 64 MBit/s (same speed on Mac and PC). Why is it so slow?

The SonicWall support technican told me that SSLVPN uses encryption and that the TZ500 only uses a single core for this. I checked the load on the TZ500 cpu cores and I never got more than 30%. So this can't be the reason.

I did this tests late in the evening and I was the only one connected to the TZ500 at that time. I also did speedtests from a PC at the customer and from my computers and always got the full speed of the internet connections.

Then I repeated the tests with several of my other customers. Of course the download rates are not comparable but they were also way too slow. What is going on SonicWall?