Antivirus apps inject themselves into the operating system at a very low level. They are more prone to cause performance problems than any other type of system modification. This is because they constantly monitor the file system in an attempt to detect malware the instant it is created in hopes of quarantining it.
Unfortunately, this is really difficult to do. Antivirus apps can destabilize the system and cause it to crash. They can also cause significant performance problems.
The read and write speed of a hard drive or SSD is usually an indicator of the physical health of the drive. But there is no way to test this in isolation from other processes running on the system. So some other app may have been attempting to use the SSD while EtreCheck was performing its write test. You would have to run more tests over time to arrive at a consensus of what the true, expected performance should be. Any single test run, including EtreCheck’s is not definitive. If it shows a major problem, then that should be investigated. But it could be an isolated incident.
The more important test is actually the File system performance test. In your case, that test failed and had to be terminated after 2 minutes. This is highly indicative of interference from 3rd party software. Even your “horrible” write speed of 295 MB/s is faster than any mechanical disk drive. But the File system test is more of a test of the operating system than the hardware. In this test, a healthy SSD and a healthy hard drive should complete in about 30 seconds.
The only way to test these issues is to uninstall the software most likely to be causing the problem. Keep doing that until the problem goes away. At that point, you will be able to measure your machine and determine exactly what your nominal performance should be like. Then, as you install (or reinstall) 3rd party software, you will be able to immediately determine if a particular piece of software is causing a performance problem. But you won’t know until you try.
