A user, Ann, is experiencing slow browsing on a website she frequently visits that displays constant pop-up ads. Which of the following is the MOST likely cause?

Browser incompatibility with a website usually results in rendering issues, broken functionality, or error messages, not typically an influx of constant pop-up ads.

The presence of persistent and constant pop-up ads, especially on a website not typically known for them, coupled with a general slowdown in browsing, is a classic symptom of adware or other forms of malware infecting the user's workstation. This malware often injects unwanted advertisements and consumes system resources, leading to performance degradation.

A website not secured by SSL (HTTPS) would typically display a 'Not Secure' warning in the browser's address bar, but it would not directly cause client-side pop-up ads or system-wide slow browsing.

An expired website certificate would trigger a prominent browser security warning, potentially blocking access to the site or requiring user confirmation, rather than simply causing constant pop-up ads.