My current one is a 2017 MacBook Pro, running High Sierra. 8GB ram. My only issues are connection-related, except for the rare instances where I get a FPS drop because of a massive number of effects going on at once (Such as if I see a Brand ult with 3+ passive explosions going off, a 7-sphere Syndra ult, and we have a lot of other things going on. Basically: Just the super extreme, rare things that'll slow down pretty much everyone). I do have the rare crash, but it's only every 1/1000 games. I did also use to have large FPS drops if I used my old (2013/14, November) bluetooth headphones, in a Discord call, while in game, although I have no idea if that issue still persists.
My previous one was a 2011 MacBook Pro, and is currently also running High Sierra, with 4GB of RAM (Being used by my brother). His only real issues are when his battery runs low (Which causes the computer to slow down all processes, so that it lasts a bit longer), and connection issues.
If you have non-ping related issues, check to make sure you don't have extra programs running, that you are using 50% or less of your maximum hard drive space (I've found Macs to slow down past that point), and that you have no bluetooth connections. In general, I find it a good idea to keep the bluetooth connection closed when playing League.
If those solutions do not work, reply, and I'll see if I can help you. I'm not a professional, but I've worked with Macs quite a lot informally.