The fastest SSDs you can get, well in the 2015-2017 MBAs you have double the speed of what you can get in a MBA from e.g. 2013-2014.
In my example, I had the original Apple 256 GB SSD what did give me about 700 MB/s read and write speed.
I have now installed the 512 GB Fanxiang PCIe 3.0 SSD and I get about 1.3 GB/s read and write speeds.
The read/write speed of original Apple SSDs in 2015-2017 MBAs also reached about this speed.
I plan to install a 1 TB Fanxiang SSD in my 2015 MBAs, I would expect the read and write speed to be around 2-2.5 GB/s, the Fanxiang SSD speed tops at about 3.5 GB/s, so if you buy an SSD that is faster it is nof no use in a MBA from 2015-2017.
You need to decide whether you go the adapter route with a selection of NVMe drives possible or if you would get an SSD that is plug&play compatible with the MBA. Most of the comments I read in the mentioned super long thread are only addressing the success stories of adapter + NVMe drives.
Regading what is compatible with these adapters, amazon has some screenshots, and there is also data material on the super long discussion (at the beginning) with what works and what draws a lot of power. The options are limited and results wary, also regarding power consumption.
You can check the details of what SSDs are tested in the beginning of this thread:
https://forums.macrumors.com/threads/upgrading-2013-2015-macbook-pro-ssd-to-m-2-nvme.2034976/
I also was tempted to buy one of these mentioned SSDs with an adapter but I decided to buy an SSD from an unknown company that is plug and play compatible and so far I was successful with my first upgrade. But it is up to you, for me personally, adapters add another factor for incompatiblitiy and stress for components (e.g. some recommend to go with short adapters others advise longer are better, I saw pictures of bent NVMe SSDs with some small adapters, so I am personally not convinced that this is a good choice for me).
Some users had success with Crucial P1 and P2 NVMe SSDs with adapters, but I have my doubt about the reliability of the P2 ones that now have much less durable QLC chips, the SSD that I would invest in would have TLC chips.
The Crucial P1 are slow, very slow, they will not give the full potential of the MBA 2015-2017:
Here is the speed max from this drive, this is mediocre, not worth the money spent.
The Crucial P2 is a bit better but as mentioned, new ones come with suboptimal QLC chips:
Also, the speed maximum of a Crucial P2 is not much to write home about, better than the P1 but not by much.
If you are willing to risk buying a not so popular brand you may be better off regarding speed.
Most of the other users recommend the SK Hynix P31 NVMe, this is indeed a better choice. The values are much better than with the Crucial P1 or P2 especially regarding read and write speeds:
