The Lightline has a very short front taper (5'), even shorter than Sci Anglers Mastery DT or WF in sizes 4 & 5. That short front taper puts you into the belly of the line with less fly line out past the tip top.
This means your rod will load with less line out than when using another true-to-weight line with a longer front taper including the Cortland Sylk with a 10 foot front taper, a 406 or a 444 Peach with an 8 foot front taper and even my current favorite, Sci Angler Mastery DT at 5.5' or 6.3 feet respectively for 4 & 5 weight lines.
Bottom line, if you are using any of these other lines and casting the same distance as with the Lightline, you should feel a difference.
I am a big fan of short front taper lines for the reasons stated above, I ordered a DT4 Lightline to try out. If I hate it I'll let you know.