Synthetic Full in the simpliest manner, is a consolidation backup of the previous full and the incremental which has ran after the last full.
The incremental are still separate to the Synthetic Fulls, and incremental could take longer for various reasons (network, read, write or deduplication/compression). Load could also be an issue.
If you want to narrow down the slowness, a review of the CVPerfMgr.log would help in identifying which hop in the Backup process is causing the slowness.
Hopfeully that helps