You've been using Marketo for a while and have started noticing that Smart Lists take longer to process than before. There are multiple filters in your Smart List ranging from really specific attributes about a person to really broad geographical information.
The best performance will come when you order the smart list filters from most restrictive to least restrictive. Since your entire database is checked against the first filter, if we can narrow down the number of leads which move on to the next, there are fewer and fewer leads who qualify for each of the subsequent filters. Many times, the processing difference will be minimal. However, as your database grows, the penalty on efficiency can grow over time. If you haven't checked it out already, take a peek over at our product documentation for smart list best practices: