DSUB9 maximum signal speed
D-sub connectors are not really any worse than 0.1" headers in terms of signal integrity. With a bit of care in terms of how you assign grounds and signals to pins, they can easily handle hundreds of MHz. For example, I have successfully passed high-speed USB signals through both kinds of connectors.

As another example, consider the standard VGA connector, which is simply the "high-density" (3-row) form of a D-sub connector &mdash; it passes analog signals with bandwidths of hundreds of MHz quite successfully.

Note that you can get D-sub connectors with custom contacts, including coaxial contacts, for the ultimate in signal integrity. These are good well into the GHz range.