I've been working on a C#/GDI graphical app for a couple years. I've spent a lot of time optimizing the drawing code. I am drawing to the screen by invalidating a PictureBox control about 10 times a second, and by leveraging subsequent OnPaint event that occurs when Windows triggers it. The OnPaint event gives me access to the Graphics object via the PaintEventArgs param.
Per frame: I draw hundreds of lines, hundreds of rectangles, and I call the Graphics.DrawString() method hundreds of times as well.
I started putting together a SharpDX project in hopes I could draw more 2D elements, and draw faster to the screen. I set up 2 test projects that draw the same 2D elements on the screen using GDI and using SharpDX. I used a C# StopWatch object to detect how long it was taking to draw all the 2D elements. So far, I have not noticed any speed improvement when drawing with SharpDX. Both GDI and SharpDX average about 20millis per draw.
How much of a speed improvement should I expect by using SharpDX? And which portion of the rasterization is supposed to make it faster than GDI?
I worked on a Windows Forms app where the "rendering system" was pluggable, and I initially wrote two rendering systems: one in GDI+, and one in Managed DirectX (a .NET wrapper around DirectX, similar to SharpDX). Because I was doing a lot of drawing of images at arbitrary scales, Managed DirectX blew GDI+ out of the water for our use case. Drawing code that used 1-pixel-wide lines was also very fast in Managed DirectX. Thick lines were much slower than single-pixel lines, because they were actually rendered as triangle strips (which can be drawn quickly by the GPU) whose coordinates had to be calculated by the CPU, and the geometry was complicated by segment joints (which were rounded, if I remember right). Fortunately, in our application we didn't need to draw curves, but those would have to be approximated by small line segments (for single-pixel widths) and triangles (for anything thicker).
It's things like CPU-based approximation and triangulation that slow a Direct3D app down. 3D games use pre-calculated meshes and make use of vertex buffers on the GPU to avoid moving data back and forth from the CPU to GPU. I don't have any data comparing speeds between GDI+ and DirectX, but these are some things to consider.