The performance of a database can be unpredictable due to various factors such as statistical query optimizers, locking mechanisms, and concurrent query execution. In traditional database management systems, changes in statistics or locking issues can lead to unexpected performance changes. However, some systems like memcached are designed to be predictable in performance. The MongoDB project aims to achieve predictability in performance by utilizing concurrent query plan evaluation and a lockless design, which helps eliminate unpredictability from locking. Despite this, there is still room for improvement, particularly in concurrent query execution.