The server get's really stressed when a new tick starts. It is both processing the missions and dealing with everyone logging on to accept new missions...
I just found this post in the forums (A Stickied FAQ in the Help section):
Q - What are nodes?
A - Amount of hosts you bounce your connection off of for each mission, 10 being the safest but 1 being the fastest.
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Q - What is balance for?
A - The higher this is the faster you will hack, BUT you will have a greater chance of being caught.
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It provides a little insight into what changing nodes & balance actually do...