Newly upgraded internet slower than expected for client machines

We just upgraded our internet service from a 4.5mbps up/down T1 solution to a 15mbps up/down fiber install.

Now my users are complaining of slow internet - the opposite of what was supposed to happen.  The most recent speed test was 2.24 up, 0.13 down.

Note this is only for client machines with DHCP.  Machines with static IPs regularly test out around the new 15/15 speeds.

We are running a Sonicwall TZ 215 firewall, and a Windows 2012 server, which serves DNS and DHCP.

If I reboot the Sonicwall, the speeds are improved for a time, but eventually slow back down again.

Any suggestions?