What is a common effect of spyware on a user's device?

Spyware is designed specifically to collect personal information from a user's device without their consent. This malicious software can monitor various activities, including web browsing habits, login credentials, and sensitive personal data, which is then typically transmitted to an external source. This unwanted monitoring is a fundamental characteristic of spyware and a primary reason it poses a security threat.

In contrast, the other options suggest positive outcomes or effects that spyware does not provide. For instance, spyware does not enhance speed or performance; rather, it can slow down the device due to its background operations. While storage might become an issue if the spyware creates numerous unwanted files, the most critical and alarming effect remains the unauthorized monitoring of personal information, which ultimately compromises user privacy and security.