Performance is not a set-and-forget setting: it needs upkeep.

The winning routine

• Measure again on a regular basis (PageSpeed / GTmetrix).
• Update WordPress, your theme and your plugins.
• Keep an eye on heavy content being added (large images, videos).
• Keep backups before every significant change.

Well done: you now have a complete method to diagnose and speed up a WordPress site, and to keep it fast over time.