The performance impact of CB Auto Actions is entirely dependent on the auto action you're using and how many. If you've 20 auto actions acting after profile update for example then it'll of course have some performance impact, but it should be unnoticeable in most cases. I can't give you any specific performance advise beyond using as few auto actions as necessary.
1) go from Access type "Everbody" to "Self" (what does this mean btw)
Everybody applies to anyone who fires the trigger. Self applies only if the User matches the user firing the trigger.
2) trying to get users into specific Custom/CMS ACLs to reduce no of actions to be fired or not
Don't understand what you mean. CB Auto Actions works off usergroups not view access levels. Use whichever Access setting makes most sense for your usage.