Calculating pi to 7 significant figures without using Math.PI
Your implementation is fine. You could add some tweaks (i += 4 instead of i += 2) to avoid multiplying i by 2.0, but this won't help much to make your program fast.

This is because you chose a simple formula, but that formula converges very slowly towards \$\pi\$.

Read on Wikipedia about efficient algorithms for computing \$\pi\$, there are several.

Or, if you want to cheat a bit, just return 4 * Math.atan(1.0).

You don't need a delta in the whole program, so only declare it where you really need it. You already did that with firstFrac, for example.

Be consistent with your chosen names. What's the difference between firstFrac and firstNum? — There is none, therefore you should use the same name in both places.

What does sumOne mean? Think of a better name for it.