The behavior described in the following KB should be fixed https://www.telerik.com/support/kb/aspnet-ajax/grid/details/horizontal-scroll-speed-is-slow-with-static-headers

In the meantime, you can use the workarounds from the article, or, if it fits your scenario, move to scrolling horizontally through frozen columns via the Prev/Next buttons the grid can provide. You can find an example of that in the second grid (Navigate through the columns by clicking on the Prev/Next buttons) in the following demo: https://demos.telerik.com/aspnet-ajax/grid/examples/columns-rows/columns/frozen-columns/defaultcs.aspx