"Syncio Plus" is a private app version of Syncio that is able to overcome the API call limits imposed by Shopify.
When a Destination store order is made, Syncio sends that update to the source store to make an adjustment. Then, when the Source store quantity is adjusted, Syncio then broadcasts that update to any connected Destination store. Each time, the adjustment has to go through Shopify's server which imposes some API call limits. For a Shopify Plus store, it is 4 calls per second.
The Syncio technical support team would need to install Syncio Plus onto your Plus store, set it up on dedicated servers, and request the Shopify MSM raise the API call limit for Syncio Plus (can be 3x normal API call limit).
Note, Syncio Plus is only available to Shopify Plus merchants.