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Placing the catalog and previews on an SSD helps a lot with overall responsiveness (e.g. anything that requires access to the catalog). However, it won't noticeably improve preview load/render times, etc. I wrote this article a few years back Will an SSD Improve Adobe Lightroom Performance? | Computer Darkroom While the references to particular chipsets and Operating Systems is no longer relevant the general findings are still valid.
The choice between a NAS and an external disk array is one that many users debate here and elsewhere. In theory, super fast access to your photos (i.e. SSD) should make a difference. However, as demonstrated above, the reality is different.
Is NAS with Gigabit Ethernet fast enough? Some argue that it's more than fast enough enough, but only for file storage. I have tried a few consumer class NAS units (eg. WD My Book Live 3TB and 4TB) but found the file transfer speeds less than ideal, particularly with large transfers where the speed seems to drop significantly after only a few hundred MB. As such, I find a traditional disk array (Thunderbolt or USB-C) but in RAID 5 rather than Raid 0 configuration to be a better solution. That being said, I am still tempted by the flexibility offered by a good quality (Synology or QNAP) 4 bay NAS (again configured in RAID5).
Ideally, you need to define the judging criteria before we can offer suggestions based on YOUR definition of "better".
Good write-up by Ian.
Another thing to consider is where the catalog is going to be located. While the images can be on a network drive, the catalog cannot. Therefore, one advantage of the simple external drive is that you can have your catalog and images on the same drive.