Malware or viruses can slow down your system. Run a full scan with a reputable antivirus program to ensure your system is clean.
Outdated or incompatible graphics drivers can cause login slowdowns. Update your graphics drivers from the manufacturer's website.
Make sure Windows is updated.
If the user profile is corrupted there may be such issue. You can create a new user account and see if the login is faster on the new account. If it is, you may need to migrate your data and settings to the new profile.
See:
Troubleshooting Slow Logins with the Windows Performance Tool Kit