Slow iCloud syncing on Windows is characterized by perpetual syncing icons and "downloading" status.

iCloud Sync is Too Slow on PC or Mac? Here’s What You Can Do Instead.

Synchronizing numerous files with iCloud on a PC can be excruciatingly slow. When I tried to copy several gigabytes of files, iCloud reported: “Downloading x,xxx items” for many hours. Often, the number of items would not change.

Slow iCloud syncing on Windows is characterized by perpetual syncing icons and "downloading" status.
Slow iCloud syncing on Windows is characterized by perpetual syncing icons and "downloading" status.

I do not own a Mac, but Mac owners have reported similar problems. Synchronization for, say, 50 GB can take days.

I suspect the issue is due to the synchronization process. The log files show that iCloud is constantly checking for changes. If you have a lot of files in iCloud, the log files can span many gigabytes.

Regardless of the reason, there are two methods I used successfully to copy files into iCloud that worked much better than the iCloud for Windows app. These strategies are below.

Strategy #1: Upload to icloud.com using a web browser

Uploading to icloud.com using any web browser is very fast. It is much faster than syncing files with iCloud on a PC or Mac.

A limitation of this method is you cannot upload folders.

A workaround for that limitation is to Zip (compress) a folder and then upload it. Then uncompress it in iCloud on an iPhone or iPad using the Files app. I assume you could uncompress it on a Mac using Finder after that Zip file shows up in iCloud.

If you are using an iPhone or iPad, there is a limit to how large a Zip file you can uncompress. On my fifth-generation iPad Pro 12.9, that seems to be around 4-5 GB. Trying to uncompress Zip files larger than that returns this error: “The Operation Can’t Be Completed: Couldn’t communicate with a helper application.”

Strategy #2: Copy the files to iCloud with your iPhone or iPad using the Files app

You can use the Files app to copy files to iCloud on an iPhone or iPad from local storage or a cloud account.

Relative to iCloud synchronization on PC or Mac, this method is speedier when it works. Occasionally I got an error, probably due to network issues. Trying again often worked.

Trying to multitask on the iPhone or iPad might interrupt the process. The device going to sleep might interrupt copying too, but I am unsure. 

If you use this method and want to ensure reliability, don’t multitask or at least keep the Files app open in Split View. Also, keep the device on and plugged in.

Which strategy should you use?

Strategy #1 is better if you do not need to upload folders.

Otherwise, Strategy #2 is better if you have an iPhone or iPad free. 

In my case, I had many folders to copy from OneDrive. But I didn’t want to bog down my only computing device–the iPad Pro–with this task. Therefore, I started out using Strategy #1. I used a Windows Virtual Machine hosted on Azure that cost about 15 cents/hour for the VM, disk space, and bandwidth.

Eventually, I switched to Strategy #2. This way, I did not have to do the extra Zipping and uncompressing steps.