Is progress forced?

Actually, as far as I know, the alien's progress is linked to yours. You might have an advantage with that new plasma rifle for a few missions, 2 maybe 3, but then they bring out a new guy. If anyone can test by not doing any research for an extended period of time, that would be great. As for the question, story progress os done, like Elessar said, by doing priority research and building priority structures. But if you're researching things while you wait you ony make those other missions harder... (POSSIBLE SPOILERS) That said the last mission, I'll bet, is the same every time, so you're gonna want some heavy artillery.

The speed at which the story progresses is entirely in your hands. You can progress it as quickly, or as slow as you would like. There were several times when I delayed progression for 5-6 months in order to build myself to a point where I can move forawrd.

The storyline progress is not forced, but it is advanced through researching certain areas that are marked with **Priority** and later on some base buildings that is the result of that research. So if you do not want to progress along the storyline, avoiding those research area will let you pace your game in how far along you are in the storyline. But the general missions will get harder the longer you take as more and more alien races will appear which in turn also means that the priority missions will get harder as any alien races you encountered might appear in those missions. To tell any further than that would in my mind be a small spoiler, but a spoiler nonetheless so just move at your own pace.