[Xcb] what are "requests that the server can process quickly" ,

[Vincent Torri]

Hey,

in the XCB Notes, i've read that sentence:
"The idea of rule 1b is that if you're lucky, you can be working on the 
quick results while the server is processing the slow request"

Question: Can someone give me 2 examples of resquests, one being processed 
faster by the server than the second ? And if possible, a more accurate 
definition of the speed of the processing of a request ?

thank you

Vincent