YouTube videos can be slow or laggy due to several common reasons including slow or unstable internet connections, outdated browsers or YouTube apps, problematic browser extensions, and outdated network or graphics drivers. To fix these issues, try the following steps:

1. Clear your browser's cache, cookies, and browsing data to free up resources and remove corrupted data that might slow loading.
2. Disable or uninstall unnecessary browser extensions that might interfere with YouTube playback.
3. Update your browser or YouTube app to the latest version to ensure compatibility and performance improvements.
4. Check and update your network and graphics drivers to maintain smooth streaming.
5. Ensure your internet speed meets YouTube's minimum requirements: 3 Mbps for SD, 7 Mbps for HD, and 13 Mbps for 4K streaming.

Additionally, some browsers like Firefox and Edge might show slower performance on YouTube compared to others due to optimizations by Google favoring Chrome, which can be exacerbated when ad blockers are used. Using troubleshoot or safe modes in Firefox or switching browsers temporarily can help identify if browser-specific issues are the cause. For playback speed adjustments, YouTube itself allows you to change playback speed manually via the settings on the video player. Together, these measures can significantly improve YouTube video loading and playback speed.