I don't know what the fixes from YouTube are. They could be good, bad, right, wrong, anything.
Your computer hardware is going to be one major factor in how well your machine performs with playback. There's no provided information about that.
The media is also another hugely important factor. Your iPhone is going to be shooting some less than ideal media. If it's high resolution then it's probably HEVC/h265, which is maybe the worst video codec you can edit with, and phone footage is also going to be Variable Framerate, which is also terrible for video editing software.
It is in .MOV format, width 1920, height 1080, and FPS 30. It doesn't lag when I edit with my laptop, but lags when I put in in my Desktop. I don't like editing using my laptop because the screen is too small so I don't get to see some of the Export options when I am Exporting. Though I am able to change the options, it is hard to understand which option I am adjusting as the screen is too small.
I have Premier Pro 2020 installled in my laptop and it can import .MOV files from iPhone, but if I use the same file and Premier Pro 2020 in my Desktop, it doesn't allow me to import because of Codec problem. Right now I have Premier Pro 2019 installed in my desktop and that video from iPhone can be imported but not useable because of lag.