Mining speed in creative inconsistent and often too fast

When holding down left click to destroy multiple blocks together, the speed sometimes goes way too fast randomly, often resulting in way more blocks being deleted than intended. I can't find out why it happens randomly but it doesn't feel the same as the quick delete that I can purposefully trigger by sprint flying forward and holding left click.

 

I have a 3mb screen recording of this issue but I'm getting an internal server error when trying to attach it.

Edit: think the attachment works now