It took me 6.5 months to lose 5 stone on MJ, so not extreme like in your OP but pretty fast! BMI of 34 or so down to 22.5. My husband was 39BMI and has lost 7 stone in same time frame.
I was in deficit every single day, there wasn’t a single day I would have gone over my calories, I upped my exercise massively, from sedentary to 10,000+ steps a day, changed my lifestyle around walking places instead of driving. I changed my diet hugely, focused on protein, reduced carbs, cut out a lot of sugar. A lot of people talk shit about how fast they’ve lost weight up though, or round it up, are economical with the truth, etc. so I would take what you see and read online with a grain of salt.
I have ended up with symptomatic gallstones (I had them already but they were were dormant for a long time and the weight loss has woken them up again!) so I need to get my gallbladder removed, but otherwise the rapidity of the weight loss doesn’t seem to have caused any issues, I feel fantastic, I’ve got energy, I need less sleep, I just feel like this is what life is meant to feel like now.
I do think some people just respond better and have bodies who are ‘happier’ to lose weight than others though, there’s all sorts of biochemical and physiological stuff that comes with weight gain and loss, and science is only now really drilling down into why it can be so different individual to individual.
I would say that three months to lost a stone when you’re 40 BMI is definitely on the slower side and I understand why you might feel disheartened. Maybe it’s less a question of wondering what other people are doing and evaluating what you are doing? Or perhaps this is just what works for you and it’s just a case of steering clear of all the social media gubbins about it and just focusing on your journey.
But it inevitably just comes down to diet. MJ makes it easier to stay in a deficit but you need to actually know what the correct deficit is for you and what food portions and type look like for being in deficit. I tracked a lot in the first month till I got a handle on what kind of stuff I should be having, then stopped, but it’s still in my head.