m977 wrote:
Disk Utility First Aid runs clean. I've already tried to reinstall macOS. Do you think updating to Catalina might help?
No, upgrading will just make troubleshooting even more difficult. A clean install of macOS (without migrating from a backup) is enough to determine if you have a hardware or software issue.
Will it be relevant to try creating a new user if I've already reinstalled macOS? A new user was made in the process and all programs and files are gone, except for photos restored from iCloud and a few programs I've downloaded after the reinstall.
Did you perform a clean install of macOS and test it without migrating anything from a backup? If you had problems with a clean install, then it means you likely have a hardware issue which needs to be repaired by Apple. If the issue only occurred after installing some apps, then one of those apps may be the source of the problem.
The Disk Speed Test gives these results: Write speed - 682 MB/s, Read speed - 2191MB/s. What could be the cause of the slow write speed and is there any way to fix it?
Hard to say. The speed test can be affected by a lot of things. An SSD can become temporarily slow after having written GBs of data in a short period of time. Usually I recommend performing a hardware secure erase on the SSD to reset it to factory defaults as it also restores any lost performance and it can also sometimes fix some other odd issues. However, only Linux has the necessary tools to access the hardware feature of the SSDs (assuming the SSD supports this feature - not all Apple SSD suppport it).
Unfortunately I haven't had much luck performing this secure erase on the 2016+ systems. I'm not sure if it is because most of these Apple SSDs don't support the hardware secure erase or whether it is due to the systems I've tried it on being locked down by a corporate management system. I'm hoping to confirm whether the 2016/7 models can be reset in this manner next week if time permits. If you are not familiar with Linux, then the only way I know is to use a paid utility called Parted Magic which is a customized Linux utility boot disk which includes an app to access the hardware feature on SSDs.
https://partedmagic.com/nvme-secure-erase/
My laptop doesn't qualify for any of the programs.
Also, the last two kernel panics didn't show the usual kernel panic message. The laptop just restarted showing the apple logo.
Sometimes the crash may not be an actual kernel panic or sometimes a kernel panic cannot be saved. Any saved Kernel Panics are found within the Console app under one of the folders such as "Diagnostics", "/var/logs", "/private/var/logs", etc.
I would suggest installing macOS to an external USB3 drive and Option Booting the external drive to see how the system works. If it works fine from the external drive (it may be slower), then the internal SSD is likely at fault. Unfortunately the SSD is an integrated part of the Logic Board on all but the non-touchbar models.
Most likely you will need to take the laptop to an Apple Store or an Apple Authorized Service Provider to be examined and repaired.