The application inspection is actually one of the leading reasons that people buy a Palo Alto product, and while it does but an increase load under the processor it is something that they are designed to allow. Further, the Palo Alto can actually be better at mitegratting a DoS attack when setup properly because it can drop packets for applications that you do not have publically available. If you are worried about a DoS attack I would recommend that you setup DoS Profiles and Zone Protection profiles on your untrust interface; both of these will allow you to not only be alerted when you have a potential DoS attempt, but will automatically start to drop packets if your set limits are exceeded. To point out as well, the PA firewall will actually stop doing application inspection if your processor reaches a certain percentage on new requests. This feature is to allow the firewall to continue to pass traffic and not 'lock up' because it's busy decrypting and analyzing the applicaiton.
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