Hi there! Are you an MS1 in an MD/PhD program now? Just want to make sure I understand so I'm giving the best answer I can.
I largely agree with the poster above. As a current second year MD/PhD student, here are a few of my thoughts (with main points bolded since this is long):
1. I really would not advise this. Many of the reasons have already been stated. It's interesting that your school only has two hours of lecture per day, but perhaps there is a lot of independent learning that takes place outside of those classes? Or labs/small groups/etc? They say at my institution that being a med student is a full time job, and I really agree. It's typically learning that takes place not just in the classroom, but outside as well and within clinics/meetings/etc that aren't always "on the schedule." Your time fills up very quickly. Even a few hours of lecture material can be dense and seem to bury you before you know it! I would worry that you would burn yourself out with such a hectic schedule, and burn out early in a career like this one is not so good.
Secondly, depending on how your institution works -- do you already have a lab that you know you will end up in? As only a first year student, you likely have not completed some or even all of your rotations yet. It would be really hard then to go and do research in a lab that you may or may not end up in. Even if you are just learning skills in some different labs, to me, you're not going to shave a significant enough amount of time off of your PhD for that to be worth it.
Finally, one of the biggest reasons that I personally would avoid this idea: One of the great pieces of advise someone gave me early in my career was a reminder that you're going to have multiple years to just focus on research. Shaving half a year or even a year if possible (which I think would REALLY require an insane amount of work in addition to classes) really isn't worth making it so that you are losing a lot of your "med school" time/experience. Again, even if your institution only has classes for 2 hours per day, there are likely many other incredible resources you can take advantage of while you're learning about the clinical side of things. Can you lead an organization? Can you volunteer at a school-sponsored free clinic? Shadow a specialty you haven't seen before? Mentor other students? I think the separation of med school and grad school time is really important because both are building you to be an incredible professional with two intertwined but very different degrees. And on the med school side, for residency purposes in the end, I have been advised that it is generally best to have more on your resume to really highlight your ability to excel in clinical practice as well. I don't personally think it's worth it to give up a lot of the experiences and learning moments you can have about being a better physician to do something that you already have years of protected time to do, if that makes sense. Of course, this is just my opinion, so take it for what it's worth! But I do urge you to consider these factors along with time/workload factors to really see if you want to do this.
2. I also would not advise this, but I am not yet as knowledgeable about this specific area yet. From my perspective now, that time in residency is another great way for you to gain some research skills and perhaps get your name on most recent publications (so good for fellowship or future lab prospects). Additionally, I imagine it would be a good chance to really try your hand at juggling patient care and lab work and to see how the two intertwine. From speaking to a fellowship director at the major research institution where I worked in undergrad, they really do look at what you did during your residency and like to see research/publications. And I think just "giving up" research for a few years could potentially hurt you in trying to show your persistent dedication to research down the line and in keeping your skills fresh? Only a guess though - again, this is a bit out of my field, but I have not heard of anyone skipping that protected research time or suggesting that we try to do so.
3. I would imagine that the answer above is very true, but I will defer on this one!
EDITED to add in things that I believe can contribute to earlier graduation:
1) Research field. Some PhDs can simply be done in less time than others. Generally, many basic benchwork PhDs cannot be done in 3 years; science tends to not cooperate and experiments can be tedious. Some students in my program have had better luck completing their degrees faster if they worked on slightly more clinical projects, or if they did let's say radiology/imaging related projects with finite amounts of data that need to be completed to obtain X amount of papers.
2) Clear communication with your program and your PI. Be willing to think through project designs and goals that you want to set for yourself, and meet with your PI to talk through them. See what they think would be feasible for you to accomplish. Talk to your program administration if you are having issues in your lab or if you are worried about the timing. They have seen so many students go through this that they will likely have suggestions for you!
3) Depending on your program structure, what you may be able to do is co-enroll: take a couple of grad school classes while you take your med school classes. I would advise this over actually working in a lab while you are in med school, since the subjects can sometimes intertwine or at the very least, you are already studying your life away anyway. This can still be very overwhelming and it wouldn't be my first choice option if your program doesn't already integrate the two because it will create a fair amount more work for you, but there are students who co-enroll in multiple programs and can accomplish a little bit of both. If some of your PhD time consists of classes, knocking those out of the way so that you are ready to hit the ground running with research when the time comes might be helpful.
Sorry for the long length --- I found this question interesting, and I hope this helps!