A few more random observations / thoughts:
- My 5K retina i7 iMac takes about a minute to generate 1:1 previews of 12 NEX7 raw files (about 25MB per file; 24MP per image). Raws are stored on an external hard drive attached via USB3. Catalog (and therefore previews) stored on iMac's internal 1TB flash drive. So I would get ~700 1:1 previews generated per hour. If you do a similar test case (that is, generate 1:1 previews for ~10 images), what "seconds per 1:1 preview" value do you get?
- Beware: when testing preview generation speed, I delete 1:1 previews for a few images and then re-generate them. However, LR won't actually delete previews if they are close to the same size as "standard previews" (as defined in Catalog Settings). Since the iMac screen is about 5120 pixels wide, the Auto setting for standard previews makes them 5120 pixels wide. Therefore, LR often won't actually delete 1:1 previews for my NEX7 images, since their 6000 pixel width is close to the width for standard previews. To be able to really delete 1:1 previews, I have to temporarily set the standard preview size to a much smaller value.
- On further thinking, it doesn't seem that gigabit versus "fast" ethernet can account for all of your slowness: even if you are only getting 100Mb/s (which is about 10MB/s), it should only take 3 or 4 seconds to read a raw file.
- Why did you start a new catalog? When I recently moved from a Mac Pro to new iMac, I copied the catalog and previews to the new machine. The grid view thumbnails display quickly. The first time I look at an image in Loupe view, I usually get the "Loading..." overlay for two or three seconds. I assume this is because the old standard preview (sized for my old 2560 pixel wide monitor) is displayed while a new 5120 wide standard preview is generated.
- Are you sure neither machine went to sleep while you were trying to generate the large batch of previews?