It way more directly influences by the bunker rates. Maybe various for a lot of the cargo it makes most sense to have the container vessel sail quicker. However, that's not taken into account while deciding between shipping lines. Further, a shipping line has loads of cargo. Some high value and time critical, some not at all.

Look at historical bunker prices to get an idea. I couldn't find a good link, but the USD/barrel is a nice indication: http://www.infomine.com/investment/metal-prices/crude-oil/al...