The speed of the SSD depends upon many factors. A brand new SSD or freshly reset SSD will write faster than an SSD which has been used a lot. When writing to an SSD the SSD has to do some preparation before the actual write takes place so the writes will tend to be slower than reads.
If you are testing the SSD you are booting from, then other apps and processes may be reading and writing to the SSD which will affect the speed test.
Every SSD will perform differently. Apple uses SSDs from various manufacturers in their computers and each brand can behave differently. The SSDs used in the current model may be significantly different than the ones in last years previous model.
Don't believe everything you see online. Unless you know all the facts about the test setup and how it was prepared, the information is only somewhat useful and relevant.
As for the battery health. This is probably just a normal drop. Battery capacity can vary a bit due to heat and also to use and the charge state. Don't worry about small changes unless the health drops significantly. If you are still concerned, run the Apple Diagnostics which will perform a test on the battery. As long as the battery condition reports "Normal" or "Good" the battery is most likely Ok. The Apple Diagnostics will usually catch any other battery issues.