I have upgraded my blog from b2evolution 3.x to 5.0.7. I have experienced a lot of performance issues. I did some performance tuning. Maybe they are useful for you. Firstly, I cleaned all unpublished comments. That makes the database much smaller and faster to run. Moreover, I went to System->General, and unchecked Log hits for every public page. That save the execution time for each page. Then I deleted all hit logs in database. For the same reason, I wish the database to be smaller. These information is not very useful. I have other analytically software to track the visitor count. I wish those tips can help you have a better b2evolution.