uMatrix is very low overhead so my first suspicion would be artificial delays introduced when some JavaScript resources are blocked; [1] is an example of this.

uMatrix is very low overhead so my first suspicion would be artificial delays introduced when some JavaScript resources are blocked; [1] is an example of this. The artificial delay does not occur in uBlock Origin because it uses a replacement script for the blocked one causing the issue. [1] https://www.theverge.com/ https://www.theverge.com/