Lightroom Mobile Sync - Extremely High CPU Usage/Sync Process Causes LR To Lag

Question

Since my other thread doesn't seem to be getting any responses, I'm pasting what I've found here. Please keep in mind I am not a beginner with Lightroom and consider myself very familiar with Lightroom's features excluding the new mobile sync.
1st message:
I'm on Lr 5.5 and using the 30 day trial of Adobe CC to try syncing one collection of slight more than 1000 images. Despite already having generated the Smart Previews, I can see my CPU crunching through image after image (the rolling hills pattern in the task manager) while doing the sync. I was assuming, since I already created the Smart Previews, that the sync of this collection would begin immediately and be done by simply uploading all of the existing Smart Previews. The Smart Previews folder of the catalog is 871MB and has stayed the same despite the CPU obviously doing *something*. As it is now, the sync progress is incredibly slow, almost at a pace like it's actually exporting full-res JPGs from the RAW images (as a comparison only, I know this should not be what it's actually doing).
Another side effect of this is that I'm basically unable to use my computer for other tasks due to the high CPU utilization.
Win 7 x64 / Lightroom 5.5
Intel i5 2500k OC'd 4.5GHz
16GB RAM
SSD for OS, separate SSD for working catalog and files
2nd message:
As a follow up, now Lightroom thinks all 1026 photos are synced (as shown in "All Sync Photographs" portion of the Catalog though all images after the 832nd image show the sync icon per image stuck at "Building Previews for Lightroom Mobile" and the status at the top left corner has been stuck at "Syncing 194 photos" for over 12 hours. Is there no option to force another sync via Lightroom Desktop and also force the iOS app to manually refresh (perhaps by pulling down on the collections view, like refreshing via the Mail app)?
3rd message:
One more update, I went into Preferences and deleted all mobile data, which automatically signed me out of Adobe CC and then I signed back in. Please keep in mind the Smart Previews were long generated before even starting the trial, and I also manually generated them again (it ran through quickly since it found they were already generated) many times. Now that I'm re-syncing my collection of 1026 images, I can clearly see Lightroom using the CPU to regenerate the Smart Previews which already exist. I have no idea why it's doing this except that it's making the process of uploading the Smart Previews extremely slow. I hope this time around it will at least sync all 1026 images to the cloud.
4th message:
All 1026 images synced just fine and I could run through my culling workflow on the iPad/iPhone perfectly. Now I'm on a new catalog (my current workflow unfortunately uses one catalog per event) and I see the same problem: Smart Previews already generated but when syncing, Lightroom seems to re-generate them again anyway (or take up a lot of CPU simply to upload the existing Smart Previews). Can anyone else chime in on what their CPU utilization is like during the sync process when Smart Previews are already created?
New information:
Now I'm editing a catalog of images that is synced to Lightroom Mobile and notice that my workflow has gotten even slower between photos (relative to what it was before, this is not a discussion about how fast/slow LR should perform). Obviously Lightroom is syncing the edited settings to the cloud, but I can see my CPU running intensively (all 4 cores) on every image I edit and the CPU utilization graph looks different than before I started using LR mobile sync. It still feels like every change isn't simply syncing an SQLite database change but re-generating a Smart Preview to go with it (I'm not saying this is definitely what's happening, but something is intensively using the CPU that wasn't prior to using LR Mobile).
For example: I only update the tint +5 on an image. I see the CPU spike up to around 30-40%, then falls back down, then back up to 100%, then back down to another smaller spike while Lightroom says "Syncing 1 photo".  I've attached a screenshot of my CPU graph when doing this edit on just one image. During this entire time, if I try to move onto edit another image, the program is noticeably slower to respond than it was prior to using LR mobile, due to the fact that there appear to be much more CPU intensive tasks running to sync the previous edit. This is proven by un-syncing the collection and immediately the lag goes away.
I'd be happy to test/try anything you have in mind, because it's my understanding that re-syncing photos that are edited that are already in the cloud should be simply updating the database file rather than require regenerating any Smart Previews or other image data. If indeed that's what it should be doing, then some other portion of LR is causing massive CPU usage. If this continues, I will probably not choose to proceed with a subscription despite the fact that i think LR mobile adds a lot of value and boosts my workflow significantly if it wasn't causing the program to lag so badly in the process.
I know this message was incredibly long and probably tedious to read through so thanks in advance to anyone who gets through it
-Jeff

Answer 1

Thanks for reporting. Just passed  along your info to some of our devs. One of the things that needs to be created (beside smart previews) during an initial sync are thumbnails + previews for the LrM app - Guido
Hi Guido,
Thanks for pointing this out. I realized the same thing when I tried syncing a collection for offline mode and found out the required space sounded more like Previews + Smart Previews rather than just the Smart Previews.
greule wrote:
Hi Jeff, are your images particularly large or do you make a lot of changes which you save to the original file as part of your workflow?
The CPU usage is almost certainly from us uploading JPEG previews not the Smart Previews - particularly during develop edits as these force new JPEG previews to be sent from Lightroom desktop, but would not force new Smart Previews (unless the develop edits are modifying the original file making us think the Smart Preview is out of date) to be sent.
Guido
My images are full-resolution ~22mp Canon 5D Mark III RAW files so they're fairly large. Even if I only make one basic change such as exposure changes, I saw the issue. By "save to the original file" I'm assuming metadata changes such as timestamps, otherwise edits to the images aren't actually written to the original file. I'm only doing develop module edits so I shouldn't be touching the original file at all at this point in my workflow.
I think it makes sense now that you mention that new JPEG previews need to be generated and sent to the cloud due to updated develop edits. My concern is that this seems to be done in real-time as opposed to how Lightroom Desktop works (which is to render a new Standard Preview or 1:1 Preview on demand, which means only one is being rendered at any given time while viewing it in Loupe View or possibly 2 in Compare View). If I edit, for example, 10 images quickly in a row, once the sync kicks in a few seconds later, editing the 11th image is severely hindered due to the previous 10 images' JPEG previews being rendered and sync'd to the cloud (I'm assuming the upload portion doesn't take much CPU, but the JPEG render will utilize CPU resources to the fullest if it can). Rendering Standard/1:1 Previews locally and being able to walk away while the process finishes works because it is at the start of my workflow, but having to deal with on-the-fly preview rendering while I'm editing greatly impacts my ability to edit. Perhaps there can be a way to limit max CPU utilization for background sync tasks?
It may help to know that I'm running a dual-monitor setup, with Lightroom on a 27" 2560x1440 display maximized to fit the display (2nd display not running LR's 2nd monitor). Since I'm using a retina iPad, the optimal Standard Previews resolution should be the same at 2880 pixels.
Thanks again for the help!