Firepower threat defence (FTD) fastpath is a feature that allows you to enable a “first phase” of access control, also called “prefiltering”, before the system performs more resource-intensive evaluations such as deep inspections. As Cisco documentation states:
Prefiltering is simple, fast, and early. Prefiltering uses limited outer-header criteria to quickly handle traffic. Compare this to subsequent evaluation, which uses inner headers and has more robust inspection capabilities.
As further stated:
Prefilter and access control policies both allow you to block and trust traffic, though the prefiltering “trust” functionality is called “fastpathing” because it skips more inspection. The following table explains this and other differences between prefiltering and access control, to help you decide whether to configure custom prefiltering.
As such, this means that any “fastpathed” traffic does not subsequently go through more inspection. However, any traffic that is not matched by prefiltering configurations willl go through the “normal” filtering and inspection process.
The above information and the related quotes have been obtained from:
I hope this has been helpful!