I would think even a five year old computer should probably be able to play an xvid AVI with the 704 x 396 resolution used by, say, Shinsen with softsubs, if you use Media Player Classic. Before I bought my current computer I was using a 1 GHz single core CPU Sony laptop to view softsubbed dramas without any problems. I think your problem is probably more related to the use of higher resolution and the H.264 codec in the mkv version.
Unfortunately, I'm aware of the reasons why anime fansubbers like to use hardsubs (fancy fonts, karaoke effects, and to avoid having them "ripped off"), so I don't suppose we'll see any softsubs other than Gattai's.
Jwinges, have you tried using your TV as the sole display, instead of cloning? That should reduce the workload on your graphics chip.
I was also thinking the Shinsen subs were great, until I looked at their subs for episode 4. I wonder if they were trying too hard to avoid duplicating the lines that other sub groups have already used. Just today I saw that yet another group -- Lunar Anime -- is coming out with hardsubs! With so many groups subbing the same episodes, it will be hard to avoid occasionally duplicating lines without deliberately picking a translation that is either more awkward or less accurate.
And yeah, I have to admit I probably come across as too harsh. I know it's loads of work to put out fansubs, and we're getting them for free. But I just find this situation -- multiple hardsub versions, each with minor but different "mistakes" -- frustrating compared with what I am used to in the drama and movie community. I'd also probably be happier if I didn't know _any_ Japanese, instead of just enough to pick out possible mistakes in the subs...
EDIT: OK, thinking back now on some of the Korean dramas I've watched, there were quite a few with much worse subs than what we're getting for Macross F. I guess I've let the otaku in me get a bit too finicky. But I'd still like to see more softsubs, so that I could, say enjoy the Shinsen subs with some HD raws.