Could Sync is performing sync, it is heavy process, please see below detailed information from help:Why is Cloud Sync slower than other cloud services’ PC applications?

Due to the below limitations, Cloud Sync might take longer to sync files with public cloud services than the official PC applications such as Dropbox and Baidu.

 

• Speed limits imposed by cloud service providers: Although no public cloud service provider has disclosed any information related to this topic, it is highly possible that public cloud servers communicate with their official PC applications through a dedicated tunnel/protocol, or impose bandwidth limitations on third-party applications that access their services on a regular basis (such as Cloud Sync).
• No incremental update: Some cloud service providers do not releases a public API for developers to track file changes. This means Cloud Sync must re-download and sync the entire file every time a file has been modified, even when you’ve only made partial modifications. On the other hand, cloud service providers’ official PC applications might be able to re-download and sync the modified bits only, reducing sync time.
• No local network sync: Certain public cloud service providers offer LAN sync, a technique that allows one client to obtain files from existing clients in the same local area network (LAN), thus significantly boosting sync speed. However, LAN sync accesses files in client computers without notifying them, which could possibly become a security backdoor. Therefore, this feature is not included in Cloud Sync.

Given the above limitations, the syncing performance of Cloud Sync shall continue to be enhanced, while also maintaining the safety and security of your Synology NAS.

DS currently does not control the sync speed, and does not have function to enhance it.

However, we will continue to try improve our service and product.