I would have checked first if this phenomenon is connected to the WAN port and/or protocols.
Please run a performance test between 2 hosts on your LAN, both with GbE ports, AV disabled. I usually use iperf for this as the same exe-file contains the server and the client.
You should see wirespeed here.
If you don't there is a problem within FortiOS such as the traffic offloading to the network ASIC (NP) being disabled. Have you reset the unit after purchase to factory defaults ("exec factoryreset") before configuring it? Beware that this command will delete all of the config.
If you do see wirespeed between LAN ports I would investigate the WAN protocol used. How do you connect to your IPS(s)? DHCP, PPPoE, static IP? There is a known gotcha with PPPoE processing in FortiOS. Cheap but dedicated WAN routers use a special chip to handle the protocol, FortiOS doesn't. It might well suck up the CPU performance if the WAN line speed is in excess of 100 Mbps for desktop models, higher speeds for multi-core FGTs like yours.
The 600C itself is very capable, with decent memory size, content ASIC (CP) and network ASIC (NP) for offloading chores off the CPU. CPU will mostly handle session setup, negotiations (IPsec, SSLVPN, PPPoE, DHCP), logging and GUI. Plus some more but will usually stay out of the way of running session traffic. That's why you would expect wirespeed performance on GbE ports. IMHO the specs on the datasheet come quite close to realworld figures.
Lastly, if you're running FOS v5.4 do update to the latest build (v5.4.6). Each patch version will fix some bugs and possibly improve throughput (while adding features which introduce more bugs...). For a used unit without contract this will be, hm, difficult but you may well ask FTNT for a contract. FortiCare will do (firmware updates, warranty extension) but of course FortiGuard would be more beneficial for you (AV, IPS, botnet IP blacklist, webfilter,...). If such contracts are no longer available from the regular price list you could ask FTNT for a 'coterm quote'.
"Kernel panic: Aiee, killing interrupt handler!"