It's not necessary that you are hitting device limit. As per the data sheet it can handle up till 170Mbps total if it's clear text only and 45 Mbps with VPN. With total 118 concurrent sessions and 10 sessions/sec new sessions during the issue, it doesn't look to be too many devices in the network. There could be huge traffic through the single session, or some traffic pattern like ALG(SQL etc) etc could be invoking the high CPU. Also, please check if you have syslog configure on tcp or any broadcast, multicast etc coming to the device. You also can take a few seconds wireshark capture from your switch and what all are coming to the firewall and if everything is legitimate.
Thanks,
Vikas