The following is from an answer I wrote on Quora.
The easiest answer is: “You just can’t cram the research.” Let’s figure it out together, assuming you’ve passed your quals, you are done with classes, and you work in the natural sciences, engineering, or computer science.
First, you need to pick a good problem, which will take some time, except if your advisor already has some good idea. Now you have a good problem, but you don’t yet know what other people have done, so you devote some time to the literature. If that is sufficient, and the problem is as simple as your advisor thought, then you make progress.
Usually your research will bump into roadblocks though. You will know that you want to get from A to B, but you would have no idea how to do that. Maybe you need to design a new experiment, maybe you need to run longer simulations, maybe something else.
Becoming a great researcher is not a matter of sit-down-and-do-it any more than wine-making is extracting the juice of grapes and then drinking it. One needs to really understand the field of choice, how the tools used differ from those other researchers are using, and then come up with a way to solve the problems. And you can’t do that in a vacuum. You have to travel to conferences, talk to various people, design and present posters or slides, work so that you can pay your tuition, and still live a life that can inspire you to keep going.
Apart from exceptional cases, reading a lot and cramming research without the feedback of social life leads to burnout, the loss of enjoyment in the science that you used to love (I have seen that happen often, though this is anecdotal of course).
So yes, it’s possible, but it can come with a cost.