Having a large number of concurrent users can lead to a decrease in performance, as the database must handle multiple requests at the same time, which can slow down access and retrieval speeds. Adequate server capacity and optimized queries can mitigate this issue. Incorrect options such as 'A well-structured schema' and 'Utilizing a primary key', while being important aspects of a database, are not direct contributors to performance issues caused by concurrent user access. 'Daily backups' can be important for data recovery, but they do not typically impact the real-time speed of database access during times of high concurrent user activity.