Caring about performance means weighing the decision carefully when choosing vendor libraries that fill up your payload. You can consciously care about performance but still be negligent to the things that affect it.
I see competent devs reach for libraries all the time when a simple solution already exists within the language. It's easy to feel like you're doing the right thing by not reinventing the wheel.
Areas I see this happening a lot in are anything dealing with file systems, HTTP request/responses, database connections, DOM/string manipulation/parsing/validation. The common theme among all these things is usually a lack of understanding of what the library/package does well enough to implement the specific use case needed.
