Performing large-scale segmentation in Automation Studio before admitting contacts into Journey Builder can help reduce the amount of contacts admitted into the journey, thereby increasing the performance of the journey.
Creating prefiltered, sendable copies of data extensions for each journey instead of using the same entry source can also help optimize journey performance as it reduces the number of contacts that have to be processed for each journey. This can help with performance as the data extension is already filtered and therefore the journey has to do less work in terms of filtering the participants. For more information, see the Salesforce Certified Marketing Cloud Consultant Exam Study Guide[1] or the Salesforce documentation on Journey Performance[2].
[1] https://trailhead.salesforce.com/content/learn/certifications/marketing-cloud-consultant/marketing-cloud-consultant-exam-study-guide [2] https://help.salesforce.com/articleView?id=mc_jb_improve_journey_performance.htm&type=5