Reducing the size of anything: websites, apps, you name it: just think about it, pay attention to shrinking easily-shrunk things, and avoid including unnecessary stuff.

Improving the performance of anything: much the same. (Caveats apply, of course. Sometimes using such-and-such a library can genuinely make things faster than hand-rolling an equivalent.)

Improving anything: actually think about things. Care about them.

(There are trade-offs with everything, of course. And time is often a major one. When you see something you want to do but don’t have time for it—“Alas for the wailing of the gulls!”)