If you haven't used a typewriter in some time the keys might feel heavy, esepecially if you're only used to using a computer keyboard, but since you already own a typewriter I doubt that's the issue. I would concentrate on cleaning the segment more and wouldn't use lubricant of any kind there. Some machines will also improve with use. Run a few pages through your Sterling and see if it improves any; I've had many machines that went from being reluctant typers in the first paragraph, to singing in tune by the time it had churned out a thousand words.

The segment is the slotted, curved bar that the typebars pivot in. It looks almost like a comb.