This is important to understand. When you agree for a contract, do not renegonatiate the offer by lowballing yourself because you found a way to do it faster. This is unlikely something that will be appreciated by management.
But now I view it as: your goal isn't to perform the best on your term. It's to perform the best on terms of whatever your management says you should do. Hopefully, that's in alignment. If it isn't, then keep on searching for a job while working there, or simply have peace with it.
The client wanted something done but they only had some time/budget to work on it. The developer looked into it and was able to get the project done. It's great.
The other way is: imagine how much your client is going to trust you forevermore if you tell them you did it in half the original estimate. The next time you slip they will happily pay you because they know you are honest. They might like you so much that they will do referral calls for other clients. Etc.
Reputation matters.