Macbook Air i7 2017 generally laggy when on wifi but runs smooth on usb tethering

Another post to update and perhaps wrap up this thread.

Before conducting tests with different accounts it struck me I hadn't tested bluetooth yet. Since it's on the same (already replaced) card as the troublesome wifi I wondered if switching that on and off would also make a difference. It didn't, and by the way wifi performance itself wasn't bad either, just that the whole computer started lagging with wifi turned on.

This reminded me of the other thread I referenced in my first post here. I got some help installing and running the now discontinued Intel Power Gadget (IPG) tool that was used there. And, low and behold, I got the same result showing the CPU being throttled severely the moment wifi was engaged, and running absolutely fine without wifi when tethering via usb. To me this looks like an issue on the logic board, and that's where I threw in the towel.

For now the Macbook runs on a clean install of Catalina, which enables me to run the drivers of a Wlan usb wifi dongle, and with that it's a nice smooth fast laptop, albeit obsolete now. Never hurts to have a spare Catalina laptop, but it's a bit of a shame of the time, money and effort spent on trying to sort this one out, especially since I wasn't able to completely do so. I might switch the new parts to another Macbook Air at some point, but where I live sellers still want incredibly high prices for them at this time. For the originally projected sibling user I got another used machine with the Monterey needed for the intended use, and it only needed a new battery.

At least there are now two documented cases of this opaque and possibly rare problem. When switching on the wifi throttles the computer it is likely a logic board problem. I hope this will help others with a 2017 Macbook Air i7 to know what they're dealing with beforehand. Thank you HWTech for the moral support and great feedback.

I have now removed the WLAN (by RealTek) via their uninstall, when seeing them pop-up I was already afraid they could become a distraction in this discussion. These are the drivers for the various usb-wifi-dongles that generally have a RealTek chipset. I only installed them áfter I noticed the problems via wifi. These dongles work flawlessly on some of my other older Macbooks, yet I found out Catalina is the highest macOS they support so it wasn’t of use here. 

I have also now removed manually all other attention points the Pro version of EtreCheck highlighted, including many things in the user library. These were the result of migrating a cleaned up (or so I thought) TimeMachine profile to this Macbook Air after the clean install. All volumes checked with Disk Utility and no errors visible fixed or unfixed. Spotlight crashed several times simply because it took ages to index as the mds_stores process ran so slow. After letting the indexing happen when tethering also Spotlight became smooth and fast.

If this problem can’t be fixed I will go back to Catalina and run a usb wifi dongle which from experience produces a very workable older Macbook. Or keep Monterey and use tethering via usb and iPhone. Yet like the poster in the other thread I mentioned at the start, for peace of mind I would really like to figure out still how wifi can have a correlation with these significant slowdowns, even after replacing the Airport/Bluetooth card.

New report attached!

Very useful feedback, thank you!

The coming week I will perform that operation via an external ssd and another user account, and will report back.

In the meantime, operation in Safe Mode is the same pain as in normal mode. Indeed the trackpad corrosion was at the usual place and not too severe. There was no easy to spot liquid residu in the case, only some dust. After that I did examine all boards with a light and a magnifying glass. No visible damage or corrosion and the keyboard is a joy to use. Since the usb tethering runs fine off that I/O board I didn't expect to find anything when checking the I/O flex cable either, and indeed I didn't. When I replaced the Airport/Bluetooth card everything around it looked like new as well. You are right the only thing not checked is the other side of the logic board. That is something I try to avoid having to take out and inspect for now, as so much is connected to it.

The most puzzling thing for me is that the Macbook runs absolutely trouble-free smooth and very fast when off wifi.

Thank you for reviewing my question, I have attached a fresh EtreCheck report. Hope it indeed has clues.

ps: I ran the same check when I turned wifi off while tethering via usb to iPhone. The outcome then was "no major issues"... And the minor issues were from the times in the day that I had used the wifi. This seems to corroborate my subjective findings, and leaves me as puzzled as before!

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