A Microsoft framework for building cross-platform mobile apps using .NET and C# with native performance and user interfaces.
I would look at the Syncfusion controls as the culprit. In a project I was working on we found that the sfDataGrid did so many things wrong that we turfed them.
The sfDataGrid called the get for a property for one of the columns about 5-8 times for each item. While this was not good the clincher was when we loaded the datagrid with 20 items it got properties so many times it took a long time to load, but the clincher was when the datagrid was loaded a second time, this time with only 4 items it did the same multiple load for each property for the 4 items but then somehow accessed the property for each item (item number greater than 4) from the previous load.
I have not used their richtextbox control but since the datagrid was not the only control with major problems I would check what it is doing behind the scenes as the reason for a long time in loading.