Shape files were originally used because they were the fastest method of displaying letters. Shape files can also be used as symbols instead of blocks. Because they use less processing power, shape files display faster than blocks. The reasons for using these files are also the explanation for why SHX files degrade AutoCAD performance. SHX files contain geometric code defining letters.
Some letters are, of course, more complex than others.
On the left is the TXT.SHX version and on the right is the ARCH-3D.SHX version. Notice more lines are used in the ARCH-3D version, so consequently more processing power will be required to display it. The fact that some SHX files contain more lines, is the reason text bogs down drawing performance. Therefore, using a font with few lines, like txt.shx increases AutoCAD performance. Release 13 improved PostScript usage a minor degree, mainly in the method used to size the font.