This is a question that crosses every first-year law student's mind – and still crosses mine three years into studying law.
When for assigned reading or during lectures, lecturers emphasise the importance of reading the full case note. However, some case notes are dreary, written in total legal jargon and oh so long! So it's common to take the easy route. A quick Google search of a case name will bring up a whole list of helpful case summaries – and, 90% of the time, these case summaries will be enough to get you by in seminars. But think about how much depth these summaries are adding to your evaluation of the case. Probably not much. The summaries tell you what happened and what was decided. Most of the time this is enough to help you apply a rule. However, when carrying out a detailed, critical analysis it is important to know why a case was decided in a certain way, what the influences were and whether was there anything else that came out of this case (as obiter) that influenced another part of the law. Several times I have found that the summaries lay out the main rule set down by the case, but the lecturer actually set the case as reading because it is a good indication of another piece of law in action.
Reading every case you are set in full would take forever, so use a study group where you can each read one case and make relevant and detailed summaries of your own. Include important judicial quotes and reasoning and try to set out clearly why the law has been decided this way in your summaries. Does it make you think about any other areas of law that you know are similar? How does the decision line up with the aims of that area? What does it say about principles of fairness? If there is a certain case in the area of law you are studying that has been highly discussed and influential in other decisions, then having a full understanding of it will gain you more marks than being able to regurgitate 20 case summaries in an exam with no full understanding of why they apply.
Speed reading is also your friend. Learning how to read quickly is a skill in itself as often you can miss information. However, with practice and plenty of reading, you will find you can skim through texts quicker. Have some aims in mind when reading cases, and you will be able to pick out the relevant information faster. You can find some interesting podcasts and tips online about speed reading, or your university library may have good information.