Low Bandwidth

We have a Sonicwall 3500 as an edge device.

The ingress bandwidth on Wan interface x1 is 33Mbps and egress is 4 mbps

The ingress bandwidth on Lan interface x0 is 5 Mbps and egress is 26 mbps

We have a 100 Mbps connection from ISP.

Does it seems like an ISP issue or something with firewall or LAN?

The users are getting only 1.5 mbps speed, how can I troubleshoot this issue.