What does it mean to establish the state of the internal node in CMOS circuit?
The switching speed of logic gates is determined by the driving strength of the transistors and the capacitances that are present on each node.

Only nodes where the voltage has to change are relevant. 

If input A is switched from 1 to 0, the node "int" is initially at 0 and as M2 is switched off it remains at 0 since M1 stays on. The transistor M3 only has to charge the parasitics of node F.

If input B is switched from 1 to 0, M1 is switched off and M2 stays on. The node "int" is initially at 0 but has to be charged to about VDD-Vth. At the same time node F has to be charged as well so the transistor M4 has to charge two nodes, which makes this transition slower: "The state of the internal node has to be established".