People who drive slowly in the left lane - especially the left lane of rural, four-lane interstates - are a particular pet peeve of mine (see here, here, here, here and here). The left lane is for passing only, and motorists who drive in the left lane without passing anyone are creating a traffic hazard as well as being inconsiderate to their fellow drivers.
But don't take my word for it; this excellent video from Vox explains why you shouldn't drive slowly in the left lane on the highway:
As the video mentions, traffic flow is improved when the left lane is used only to pass. The video also mentions that crash rates are lower on Germany's autobahns than they are on American highways - even though many sections of the autobahn network have no maximum speed limit - because German drivers are very observant of the "left lane is for passing only" rule. As somebody who recently spent several days driving on German and Austrian autobahns, I can confirm that this is true; slower traffic ALWAYS stays to the right, and traffic flows wonderfully as a result.
The bottom line is this: the left lane is for passing only. It doesn't matter if you're driving the speed limit; if you're in the left lane and you're not passing anyone then you need to move over. If you don't understand or refuse to obey this concept, then you probably shouldn't be driving to begin with.