In general, having two monitors should not slow down your computer. In fact, using multiple monitors can often be beneficial and increase productivity, as it allows you to see and work on more information at once.

However, there are a few factors that could potentially cause your computer to slow down when using multiple monitors:

1. Graphics card: If your graphics card is not powerful enough to handle multiple monitors, it may struggle to keep up with the additional workload, resulting in slower performance.

2. Display resolution: Higher resolution monitors require more processing power to display the additional pixels, which can slow down your computer if it is not powerful enough.

3. Other programs and processes: If you have other programs and processes running on your computer that are consuming a lot of resources, this can slow down your computer and affect the performance of your multiple monitors.

Overall, if your computer is powerful enough and you are not running too many resource-intensive programs, you should not experience any significant slowdown when using multiple monitors.

If you are experiencing lag on your second monitor, you can try the following methods to improve the situation:

1. Check the refresh rate of the monitor: A lower refresh rate can cause the monitor to have slower response times. You can try setting the refresh rate to the highest setting in the monitor's settings to see if it improves the situation.

2. Check the performance of your computer: If your computer is not performing well, it could cause the second monitor to lag. You can try closing some unnecessary programs and see if it improves the situation.

3. Check the connection method for the monitor: Using a slower connection method such as VGA or DVI could cause the monitor to lag. Try using a faster connection method such as HDMI or DisplayPort to see if it improves the situation.

4. Update your graphics card driver: If your graphics card driver is outdated or damaged, it could cause the monitor to lag. Try updating the graphics card driver to see if it improves the situation.

5. Restart your computer: Sometimes, restarting your computer can fix the problem. Try restarting your computer and then try using the second monitor again.

If none of the above methods solve the problem, it could be an issue with the monitor itself or the cables you are using. You may want to try using different cables or replacing the monitor to see if it resolves the issue.