Why is website speed so important?
Visitors have no patience or time to deal with slow websites and poor performances. They will leave website like that in no time, even before the page is 100% loaded. That means website speed has a strong impact on a number of sales made, a number of ad clicks (impressions as well), and overall happiness of your visitors and your happiness as well, because: happy clients -> more sales/conversions -> more money for you.
There is a big number of available optimization plugins for WP, but there are few well-known plugins that all WP websites should have installed: