There are times that it can take several hours for my files to show up after adding them using a different device. It’s on Microsoft’s end I’m assuming. A delay of some sort.
If it’s on Microsoft, you or your administrator(s) will see the issue in their service health warnings.
In recent times I’ve caught a lot of my users not updating their client software and on one of my own Macs the upload sync was really slow prior to the OneDrive client update and a reboot.
On occasion it’s been slow with a mobile device when one or the other of WiFi or cellular were not happy. It can also seem or be slow if you’re on a WiFi network with traffic shaping. More and more public and commercial networks do that for reliability. As an example, our business and store locations get 400+ people a day. More than a /24 subnet. Short DHCP leases and traffic shaping are a must.
I have sites with 250 and 300 Mbps symmetrical fiber and SLA. Not whatever BS a home ISP might advertise for performance. In that scenario it is rare to ever see the problem as Microsoft’s cloud unless they have an active service health warning.
For mobile, there are times when just using one of the phone’s radios solves it - use your cellular only or WiFi only.
For desktop more and more I just use Office 365 web only. That’s how most of our employees are licensed.
Good luck.