Any ideas on a build to help Karthus power up faster?

I've loved Karthus since my earliest days of League, and lately I've been playing him a lot (trying to git gud enough to hit Mastery 7), and I've been finding myself kind of wishing Karthus wasn't so slow to pick up speed. Wondering if I should adjust my build for something that racks up kill power faster.

My core items tend to be the usual, . Those two are nearly always what I try to build first. From there I sometimes build for the fun of the AP boost it provides, but in general I find as Item 4 and 5 to be more reliable. Depending on game needs I'll finish the build with either or if I need MR for some reason occasionally. Those last three may be bought earlier than Item 6 if needed.

What I seem to be seeing a pattern of is Karthus needing almost half the game to get strong enough to become a threat, while a lot of other champions start out strong out of the gate. I'd like to speed up his entry into Threat Zone.

Should I abandon and get something with more immediate impact? Rush sooner maybe?

Any advice is helpful, thank you.