First step would be to check the execution plans for both of these.
Make sure you are getting the same plan in both cases. If you are getting different plans, this is likely what is causing the difference in performance.
Might want to watch for blocking as well. And, depending on how busy your server is, it could be resource constraints (CPU, memory, disk I/O, etc).
I expect that adding the trigger is not what is causing it to be faster but more causing something somewhere to perform differently which makes the query run faster.
The above is all just my opinion on what you should do.
As with all advice you find on a random internet forum - you shouldn't blindly follow it. Always test on a test server to see if there is negative side effects before making changes to live!
I recommend you NEVER run "random code" you found online on any system you care about UNLESS you understand and can verify the code OR you don't care if the code trashes your system.