What Val said. A warning from the third edition of my book about fast_accel:
While using the fast_accel feature ensures highly efficient handling of the
matched traffic in the SXL path, doing so ignores portions of your security policy and
can disable almost all firewall inspection of that traffic. As such a variety of bad things
can happen inside traffic streams that have been essentially whitelisted by this feature,
and a careful risk analysis is necessary before using it. It is NOT RECOMMENDED to
use fast_accel if one or both of the systems involved are not trusted and/or under your
organization’s direct administrative control.
As discussed in my CPX 2020 speech Big Game Hunting: Elephant Flows, typically fast_accel is used in the context of elephant flows (heavy connections) to make them go faster and keep from stomping on "mice" connections. Note that there is an alternative solution from R&D that allows the processing/handling of elephant flows to be "spread around" more than one worker core, thus allowing them to go faster without limiting inspection of them with fast_accel. See my preso for more details about this new feature and how to contact R&D to obtain it.
Attend my online "Be your Own TAC: Part Deux" CheckMates event
March 27th with sessions for both the EMEA and Americas time zones