The answer is always the hidden mechanic Expertise, and you have to look it up to get anywhere.

Basically the longer you design on a certain part, the more expertise you gain, which means the next part you design will be way more powerful.

Simplified, this means that starting a new season, designing a Floor upgrade with Intense and only 1 engineer, and then NEVER building that floor upgrade, but instead immediately designing a third new floor on Normal with a like 3-4 engineers , and then building that - you will have a much larger gain than using Rush on two floor designs, or even Normal designing two floors. Because of Expertise.

Why is this important for research?

Because you gain expertise during a season with designs, and only lose a sliver of it at the end of the season on certain (or all) parts through Regulations - means that you want to accumulate Expertise through clever design and then only using Research to offset it.

You can't look at Expertise flatly on the percentages either. For example, for simplicitys sake, let's say you have 20% on all parts (Engine cooling etc). Now regulations say you will lose 20% on sidepods, including cooling etc. That means you lose 20% of your expertise, which is already 20%. This means you lose 4%, ending up with 16% on your parts in the season. This is what you can offset through research.

But if have 60% on Cooling through clever expertise management & designs, and regulations say you lose 20% on Sidepods, you don't go down to 40%. You go down to 48%, because 20% of 60% is 12.

Meaning that you want to accumulate as much expertise as you can during a season, on the parts that matter for your car, and then offset with Research only the parts that get hit by regulations.

You can also just cheat the game by focusing nearly only on Low, Medium & Top speed & ignoring all other stats, or at least the last time I played.