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ASA5520 -- Which is more CPU intensive? Deny ACLs or Object-groups?

I need to block all traffic entering our network from an MPLS interface destined to a handful of "protected" subnets (mostly RFC 1918) but allow it out the Internet interface...Which of these is least cpu intensive, or is there perhaps a better way to do this?

1)  Make an object-group that contains all of the subnets I want to allow traffic to (all non-RFC 1918 addresses) and let the implicity deny at the end handle blocking traffic to the "protected" subnets, or
2)  Make an object-group that contains all of the RFC 1918 subnets and place an explicit deny ACL above the "permit any any" ACLs?

....also....is it generally better to use object-groups or separate ACLs, strictly from a performance perspective?

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j4llenAuthor Commented:
I just realized what a stupid question this is -- I can much more easily filter out the blocks I want to deny than allow all the blocks in between....I just don't like the idea of Deny ACLs since their placement in the ACL stack matters, and its easy for somebody to miss something later down the road....I guess I was wondering if there's a better way to do what I'm trying to do....

anyways, I'll keep this open to answer that and my other question:

is it generally better to use object-groups or separate ACLs, strictly from a performance perspective?
Personally, I like using object groups because you can add/remove individual networks/hosts with a couple mouse clicks without having to re-create the access-list each time.
Yes, placement matters, but you only have to place it once using a group.
unless you have hundreds of entries, I wouldn't worry about CPU on the 5520. It can take it and not even breath hard.
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