Premature optimization is the root of bugs that you need to handle for 18 years.
Optimization is the process of improving the performance of the program. The two common things that one try to improve is the speed, so it runs faster, or memory, so it use less of it. Often it is possible to trade one for the other, for example to calculate things in advance so the program do run faster but consume more memory. The process of optimization is made up of the following three steps.
Profile to find the hotspot
First one need to find a area to improve. The best one is the one that has the biggest potential to improve the overall performance. To do that one can use external or internal profiler. A internal profiler is something that one has built into the game and an external is another program that measure it's performance. In the end the profiler gives a measure of the current performance of that area. Common units to measure performance in is seconds (for speed) or bytes (for size). Depending on the scale unit prefixes such as milli or mega is common. When measuring speed one should use the unit seconds and not Frames per second.
Do whatever is needed to improve the speed, size or whatever of the hotspot.
Run the tools (profiler, memory trackers) again to see if the performance has improved. If not try again :).
Build Time - Make the game compile faster.
Code Binaries Size - Make the binary file as small as possible.
Development Time - Improve the iteration time of the development of the game.
CPU Speed - Spend less time on the CPU.
GPU Speed - Spend less time on the GPU.
Memory - Use as little memory as possible.
Cache - Use memory in a way that your CPU like.
Content Size - Get the size of the game down.
Rules of optimization - 2018
The Elusive Frame Timing - 2018
Optimizing software in C++ - 2014
Looking For a Good Sort - 2014
Don't Help the Compiler - 2013
A Profiling Primer - 2013
Optimizations that aren't - 2010
OPT#1:Profiling - 2020
Optick: C++ Profiler For Games
Optimizing Software Occlusion Culling - 2013
Performance Analysis and tuning on modern cpus - 2020
DoD book is good.
Otherwise, just go with simple stuff, and use a good profiler like Superluminal, to get the low hanging fruits.
You can go with the VS builtin profiler, cachegrind, et
The Art of Writing Efficient Programs