This benchmark tests several approaches to text rendering in WPF against each other:

TextBlock: a WPF Canvas with TextBlock elements
FormattedText: using the DrawingContext.DrawText method
TextLine: using the Media.TextFormatting namespace
GlyphRun: using the WPF GlyphRun directly
	disadvantage: no automatic font fallback -> no unicode support

Direct2D: using DirectWrite
WinForms: using the WinForms TextRenderer class
GDI: using GDI via P/Invoke
GDI+: using the System.Drawing.Graphics.DrawString method
	disadvantage: accurate measurement of font width seems to be impossible (MeasureString is not good enough)

The speed of the WPF methods is pretty consistent on most machines, ordered like this:
GlyphRun (fastest)
TextLine
FormattedText
TextBlock (slowest)

However, even on fast machines like my Core i7, none of the WPF methods provides a fluent animation.
It seems as if most frames (counted by number of OnRender calls) are not drawn to the screen.

"WinForms" is similarly slow to the WPF approaches (maybe a bit faster on older machines).
Direct2D, GDI and GDI+ all beat WPF hands-down, delivering a fluent animation even on my old notebook with
Intel onboard graphics. The exact order of these seems to depend on the hardware.

Update: I have since discovered that the WPF DrawingVisual class can be used to
significantly speed up WPF text rendering.
I did not update this benchmark to take advantage of DrawingVisual.
Take a look at AvalonEdit's VisualLineDrawingVisual class for details.