Gajet, you are asking us to tell you about all of the millions of possible cases where modifying your C++ code can yield a performance improvement.
A performance improvement at the low level you're describing on one type of hardware is not necessarily a performance improvement on another type of hardware.
For example, your C++ code running on a processor with a 12MB L1 cache might benefit from using a method that consumes a great deal of memory, where that same method yields slower code on a processor with a 512kb cache.
Judging by your question, low level optimizations like this should be the least of your priorities.
If you want to write fast code, first consider learning about different algorithms.
Here's a book on the subject:
The algorithm you choose will often affect your runtime speed much more than any low level optimizations of particular operations in your C++ code.
My program written in Python might run 100x slower than your program written in C++ for small amounts of input data. However, if I choose a better algorithm than you (e.g. heap sort), then my Python code will always run much faster than your C++ code (e.g. insertion sort), no matter how well you optimize it, when the input data size is large.
If you are just getting started with programming (i.e. you have less than 1000 hours of experience) then by far your priority right now should just be building things that actually work, efficiently or not. Otherwise, either start reading about alrgorithms, or profile your code and come here with a specific question about how to make a particular solution go faster on some particular hardware. We can't address all of the millions of possible cases for you here in advance.