SPI was designed to shorten the response time of firewall decisions in the router. Disabling this option will not affect firewall functions. Stateful Packet Inspection (SPI) mainly consumes memory but not CPU resources. Enabling SPI will reduce throughput performance for a single session.
Try to use to multi-sessions to improve performance. The router may not reach top speed with only one session.
Do not select this option if you require the higher transmission speed.