Is your Windows 11 PC slow at startup, while browsing, or only in one app? Identify the pattern first. The safest performance improvements use tools already in Windows.
Check the basics
Restart the PC, then open Settings > Windows Update and check for updates. In Settings > System > Storage, check the free space on the system drive and review Cleanup recommendations. Read each item before deleting anything, particularly Downloads.
Reduce startup load
Press Ctrl + Shift + Esc and select Startup apps in Task Manager. Disable an app you recognise but do not need immediately after signing in. Restart and check whether this helps. Disabling startup does not uninstall the app.
If it is still slow
Open Task Manager while the problem occurs to see whether a particular app is using a lot of memory, CPU or disk. Avoid registry cleaners and random speed-up tools. On a work PC, ask IT before changing security or management software.
Further reading: Microsoft Support: PC performance.