I feel the need to address that comment. Yes, tip vortex is a function of speed, and it applies to any airplane making lift. For an airplane flying level, as Will said, the slower it flies, the more tip vortex generated. (Ask me to tell you the story of my boss and the Thunderbirds). I was curious to see what happens for an airplane flying through a turn, so I did the cypering while waiting for the weather to abate yesterday at Arlington. I figure that vortex energy per unit length along the airplane's path is equal to induced drag. Energy is force x distance, and drag is a force. Does that sound right? I calculated induced drag in straight, level flight, and in a manuever (a loop corner, for example) disregarding gravity. I used formulas I had in my head for "centrifugal" force and induced drag coefficient and definitions of lift and drag coefficients. I can type them here if anybody is interested, but it's kinda a waste of effort, because probably nobody will read this. The upchuck was that induced drag (vortex energy per unit airplane path length) is inversely proportional to airspeed squared, as Will said, for straight, level flight. In a maneuver, though, induced drag is directly proportional to airspeed squared.
The formula I calculated for induced drag in a maneuver is kinda interesting. It's (2 m^2 V^2) / (pi A e S R^2 rho), where m is airplane mass, V is airspeed, A is aspect ratio, e is the efficiency factor in Serge's plot above (higher for Flite Streak wingtips), S is wing area, R is loop radius, and rho is air density. So fly slow and go easy on the corners on a calm day, particularly if it's hot or high.
You guys check those calculations.