Tried testing with only returning a number, resulted in 4000+ ms

It's not really desired situation, but possible. This time is a sum of the test suite startup time, generation of random inputs, and running the reference solution. If there's many cases and long arrays, test suite can take some significant time even with empty solution.

add 2 more lines, resulted in 7000+ ms, is this normal?

This is normal if the two lines you added are terribly slow. I added like 10 lines and time increased for about 0.5 second.

that's what i need)