Final answer:
A known issue with antimalware software is that it can slow down system performance due to its resource-intensive nature, despite being crucial for system protection.
Step-by-step explanation:
The serious problem with antimalware software often cited is slowing down system performance. While antimalware tools are crucial for protecting a computer from viruses, malware, and other cyber threats, they can be resource-intensive. This can lead to slower processing times, longer boot periods, and delays during file operations. However, the other options listed, such as overprotecting sensitive data, improving overall system security, and enhancing user experience, are generally considered to be benefits of using antimalware software rather than problems.