Most important part of restoration?
What is the most important part of making a car accelerate like new?
Make sure the brakes are working equally as well.
The boring stuff like brakes, tires and suspension always come first.
Making sure that the fuel system is working right and that the ignition system is working right and that the engine has proper clean lubrication
After reading your headline, I was gonna say "rust mitigation in the frame."
But you are asking specifically about acceleration/engine performance. For that, not to be flippant, but the answer is "everything." Fuel, air, spark, compression, control systems... it all has to work properly to get best performance. Kinda like asking "which piston is most important?"
To give you a more helpful answer, it might be good to know what you are restoring - year, make, model, engine option, current condition. Cars tend to have common problems that come up regularly as they age, and it makes sense to focus on the most common issues. Or, a car that has sat for years in a field might have different issues than somebody's high-mileage daily driver.
There are a lot of experienced restoration guys around this forum, so the more information you give, the better the answer will be.
I made new brakes but without new OEM coils and igniter chip.
Also didn't change the pump and regulator, or the ECU.
If you get the fuel system wrong, you're gonna have issues.
If you wire up the ignition wrong, you're gonna have issues.
If you don't get the timing right, you're gonna have BIG, LOUD, EXPENSIVE issues.
If you don't get the tune right, you're gonna have issues.
Just too much to it to take one thing and treat it as a silver bullet solution.
Start with simple stuff first. Air filter, spark plugs etc. Clean the intake. Run some diagnostics.
Hard to help you without any context or vehicle information.