Hello and welcome to the forum.
To answer your title with as much detail as is possible... based on your supplied detail...
"Because something is slow" 
For the expanded version of that answer we would need to know a lot more about what your node-red is running on (although that looks like Windows 10, clarification of details would be nice), when this started (EG. after you changed something?), what have already tried to troubleshoot, more detailed logs, and so on...
Also, have you searched the forum for similar issues? Here is an older reference
