Bar speed varies among lifters of all levels of training advancement with the explosive abilities of the athletes. Some people are slow when it's easy, and some people are fast right up to failure. Gauging an effort by bar speed requires that the coach be familiar with the lifter he is watching, and this prevents the use of bar speed as a proxy for RPE, your intended purpose with this post. I'm sorry, but some things require the experience of a coach who is familiar with the lifter to be evaluated properly, whether in-person or online.