The key tenet of the very low tension theory is greater snap back on the mains. Higher swing speed allows more force on the strings = greater displacement. I would argue that slow swing speed won't generate much spin either way, and no spin means you will be sling-shotting the ball on low tension and back boarding the ball at high tension. As for racket angle, the more acute the angle to the ball, the more the ball displaces the mains vs sling-shots off of them. This contributes to why low low tension works on sw and western grips because you are sling shotting the ball around itself instead of back across the court.