Global variables in JavaScript, while useful, can degrade browser performance if not managed properly, leading to increased latency and memory issues. JavaScript developers often face challenges with global variables due to their persistence throughout the code's execution, which can result in inefficient memory usage, especially when handling large data caches. Accidental global variables, which occur when undeclared variables are automatically treated as global, further complicate this issue. To mitigate these problems, developers are encouraged to prioritize local variables and utilize techniques such as setting variable values to NULL after use and employing the "use strict" directive to prevent the creation of unintended global variables. This approach helps maintain code efficiency and prevents common pitfalls associated with the misuse of global variables.