People will jump down your throat for this but there's something in it.
I think it depends on what you're like, and there will always be the person who is all "I have three sets of twins and run a merchant bank and am a sing parent, you just need to get up earlier" etc - but for most of us average mortals, your ability to be 'good' at work definitely takes a nosedive post kids I'd say.
I remember when I'd just stay late when I was busy. When I'd just sit and work until I was caught up and then dawdle home when I felt like it, sometimes stayed until 8 pm then grabbed a late dinner with DP. Or I could pull an all nighter if I'd let a deadline slide. When I went through a spell of getting up at 5 so I could go and swim before crossing London to my then job, a 1.5 hour commute each way all told. My life was basically only ever as difficult as I chose to make it, because I had SO MUCH TIME and SO MUCH FREEDOM and I had no idea.
To be even adequate at work now I have to be 100% more organised. I can't get up and go out earlier because the kids don't go to nursery/school before 8.30. I can't just stay late if they need me, the need picking up from nursery/after school club. I can't just get on with work once I get home, there's dinner, bathtime, bedtime, a few maintenance housework jobs and before you know it it's 8 o'clock. Pull an all-nighter? Well it can be done but it's quite hard to get your eye in when you get summoned to breastfeed the little one at midnight, 2 am and 4 am. Or randomly refuses to settle at all until after 10.
So for the hours I'm at work, to be considered really good at my job, I have to be a super-organised, hyper-efficient ninja. On 5 hours broken sleep and almost zero time for self care.
i wouldn't have it any other way. Like people say, I chose this life and it is brilliant. My days are filled with work and with the needs of small people - but also their hilariousness, their development, their little triumphs, their love, their lives. I am happy to be considered a bit shit at work for a few years more. I used to worry a lot. Now i just think if I'm not in disciplinary, if my colleagues seem to like me well enough, and I don't fuck anything up too much, good enough is good enough for me. I'd rather be good enough at work and good enough at home than have either end of the scale slide down too far.
BUT I had absolutely no understanding of this before I had kids, and I know my childless colleagues (probably) have no idea. I remember managing mothers before I had kids, and while I was always accommodating and flexible I do remember thinking surely kids couldn't be ill THAT often, and surely you have SOMEONE else who could watch them etc - fucking idiot. No idea. I can only apologise to my former reports for my uncharitable, utterly clueless thoughts.
Everyone has their challenges of course, it's not just kids. And some people have other challenges AND kids. I'm lucky to be in a team now where we're all very upfront about our respective challenges (kids for some of us, elderly carer for one of us, chronic fatigue and IBS for another of us) and support and accommodate each other. It's the way humans should be with each other, rather than being all sniffy and "you've made your bed now lie in it!" After all it will come to all of us at some point to be overextended - be it kids, caring, illness, mental health struggles, bereavement, a suddenly unmanageable workload - so while we're comparatively freed up, we should pay it forward, on the basis that it won't be long until we too will need some grace extended to us!