Why would smart previews take longer to build from one SD card to another?

Ever since the update the whole catalog has been buggy I've found. I've started a new catalog with a new computer and it seems to be running ok. Problem now is with the syncing, I'm getting 10 pics synced every 20 mins with 700mbps internet speeds. 

Can someone validate my anger with adobe support? Its so bad it makes me feel like I'm living a nightmare when I'm talking to them. Zero help, constant transferring. I've gotten way more support on this forum. 

Just did a bit of an experiment. I took camera A (problem camera) and put the SD cards that normally go with it into camera B (the control variable). After importing a gallery of about 500 photos on each I'm seeing the SD card from camera A exhibiting the smart preview problem, even though it was shot with camera B and even though the files are copied to the local drive before the previews are built. It's the same problem - about 1 preview built for every 4-5 on the other card. This obviously leads me to believe it is the fault of the SD card - which is still strange because the files are local for the building of previews - but I did another experiment where I used the second SD card from each camera and the problem persisted even with Camera A's second card. I can get new SD cards no problem, but now my question is was it the camera that possibly corrupted these cards? Will I be up against the same problem once I slap brand new SDs in camera A? I know there is probably only one way to find out, which I will and shall report back. 

PS. Another strange problem with LRC that may or may not be a clue. Everytime I open my catalog I check for missing files and lately there are always 10. Exactly 10 that were originally sourced from the Lightroom Sync folder inside the catalog folder. I do not import images here, nor do I download images via the cloud. The photos that are missing are not specific to one SD card or the other though. Just thought it was strange for these problems to be coinciding. Eventually I will be getting a new Mac to eliminate the final variable. 

This should be a bit far fetched, but

LrC /Preferences/Presets/ RAW Defaults, not set to a specific cameras serial number perhaps?

Thus, the cause is the camera.

By @shotbydna

Are you sure this camera is not set to uncompressed raw while the other is set to compressed raw? That would explain it...

Are you perhaps by accident using "add" instead of copy? If using add, you would be limited by the speed of the sd card and reader. Also, are you sure you need smart previews? Smart previews are only useful if you want to edit your images while they are not online. They offer basically no (or actually negative since you are waiting for them to be generated) speed advantage or other advantage when you have your images actually available on modern machines.

@shotbydna , You stated "I am importing two SD cards into lightroom classic and the smart previews take significantly longer -"

Please indicate what option do you use on import, Copy, Move, or possibly Add?

Double check. are you sure that both cards have RAW files. Some cameras with two slots can have:

• All photos first to slot 1, then Slot 2 (sequential)
• All photos to both slots (backup)
• RAW to slot 1, JPEG to slot 2

Also, be aware that on some cameras, if shooting RAW+ JPEG, with JPEG going to slot 2, in camera pano, any toy filters, any multi frame creating JPEG, and video, may go to slot 2, so slot 2 may have more files. (mind you only the video files will eat lots of space)