What are some bad coding practices

This will help you avoid bad work practices by employees

Since you are a team leader ...

First ... did you have coding best practices? If not, the bad code is your fault.

Second ... are you doing a formal or informal code review? If not, then again, the faulty code is your fault.

Third ... is the code actually bad practice, or is it not your preferred way of doing things? Since you said, "... because it can slow the app's performance," that seems to mean that it isn't broken, just not the way you want it to be.

Regardless, to answer your question, NO ... don't make the changes yourself. Talk to PM and if there is time and a real need, have the developer who wrote the code make the change and explain why it is needed. You sound like the "perfectionist" programmer. So, honestly look at the code and yourself and ask if you just don't want things to be done your way or if it really is bad code. Know that if you don't choose someone's job just because it isn't done the way you would, a surefire way to lose the respect and desire to listen to you.

Good code and a quality product are not your only company responsibilities. You should also educate and move your team forward.