The relevant CheckPlayerSpeed code.

gistfile1.cpp

        // Check player speed
        // only consider (x,y) motion for speed and position correction
        if (master->PlayerInfos != NULL && master->PlayerInfos->CheckSpeed && CheckPlayerSpeed && fabs(movVector.x)+fabs(movVector.y) > maxDist)
        {
                double          movNorm = sqr(movVector.x)+sqr(movVector.y); // already done if (entity != master) but here is a rare overspeed case

                if (movNorm > sqr(maxDist))
                {
                        if (VerboseSpeedAbuse)
                        {
                                nlwarning("CWorldPositionManager::movePlayer%s: limited speed (movNorm=%.2f, movMax=%.2f, maxSpeed=%.2f)", entity->Id.toString().c_str(), sqrt(movNorm), maxDist, limitSpeedToUse*0.001*SecuritySpeedFactor);
                        }

                        movVector *= (maxDist / sqrt(movNorm));
                }
        }