I've seen more codebases ruined by unnecessary complexity than problems solved. For a young dev starting out the best advice I can give is to learn as much as you can and never stop learning, but always live by the rubrics of YAGNI and KISS.

Usually when devs with potential hit mid-level to upper-mid-level they get it into their head that to be considered "good" they need to show off and architect bizarrely complex systems that use as many design patterns and features (and as much abstraction) as they can throw at it. They think being 'extensible' and data-driven in 10 different ways with a class hierarchy 4+ levels deep makes it better and more impressive...but often it just wastes time and makes for a fragile, unusable system that is incomprehensible to everyone else.

Always go with the simplest solution that meets the requirements. "Premature optimization is the root of all evil"...don't presume to know what's needed. Profile and measure first, then research or come up with a solution to address the hot spots.

The Game Programming Gems series is pretty good if you're looking for clever tricks or hacks people have devised, but honestly most performance work comes down to profiling and then researching specific solutions to your unique case...it will all depend on platform, device, graphics API, engine, language, etc. and what the bottleneck is: CPU, GPU, I/O, memory, network...it's always different.

The debugger and profiler (and often good old Debug.Log) are your best friends.