To increase security and performance, Windows 10 in S mode runs only apps from Microsoft Store. If you want to install an app that isn't available in Microsoft Store, you'll need to permanently switch out of S mode. There's no charge to switch out of S mode, but you won't be able to turn it back on.
Is there a downside to switching out of S mode?
Once you turn S mode off, you can't go back, which could be bad news for someone with a low-end PC that doesn't run a full version of Windows 10 very well.
Does switching out of S mode affect performance?
No, there is no big difference at all.
Do I need to be in S mode?
S Mode sounds limited, and that's the point. If you only need a basic Microsoft Edge web browser, Microsoft Office applications like Word, and anything else that's available in the Microsoft Store, you should try using the PC in S Mode. The S Mode restrictions provide additional protection against malware.