Thanks for contacting Syncfusion support.
We are also facing this performance issues in EJ2 Gantt control, for this we already logged a feature report as Virtual scrolling support in Gantt. Using this support we can load large tasksets without any lag.
The status of this feature can be tracked from below feedback link.
We will implement this feature and include in our upcoming Volume 1, 2020 main release which is expected to be rolled out in the month of March 2020.
Until then we appreciate your patience.