I'd like to make one simple change to the game - slow down research by a factor of 2 or more. What is the best way to do this?
There are a few different ways to do this. The problem is that it's not always obvious how far each change extends; for instance, the Base Research Rate mentioned above; while it'd decrease the base value from 1 per population to 0.5 per population, I'm not sure it'd decrease the additional base from the Library (+1 per 2 population), so you'd only see a decrease of 1/3 instead of 1/2. I'm also not sure whether that Base Research Rate affects flat +research bonuses, like the +3 from the Palace or any other +research buildings you decide to mod in.
Another way to do this would be to change the effective costs of the techs themselves. You don't actually need to go through and edit each tech to do this; an easier way would be to edit the CIV5Eras.xml file. Each era has a research cost multiplier, used whenever you start a new game in that era (so if you start a game in the Renaissance Era, it might multiply the costs of every tech by 0.4 to compensate for your lack of large cities). So all you'd need to do is multiply each of those multipliers by 2 (or if you only ever start in the Ancient Era, only that one), and voila, all techs now take twice as long to acquire.
Personally, I went with a more complex solution for my own mod. To me, the research pace at the earlier eras is just fine; it's the later eras, once you get the stacked +50% bonuses from the University/Observatory/Public School and the +100% Research Lab, that everything falls apart. So I just edited all of those (to 30/20/40/50, respectively, with 1/3/1/0 culture added to each) downward. Also, the Library; it's +1 per 2 population, but that means that it's the only +research that ISN'T purely additive. That is, a Library multiplies your research by 150% regardless of those other buildings (while the +50% of a Public School might only take you from 200% to 250%, effectively a 25% increase). So in my mod the Library was reduced to +1 per 3 population (and +1 culture).
End result: in the core game, the full set of research buildings gives you 5.25 research per population in the Modern Era. In my mod, it's down to 3.2, so you get a slower Industrial/Modern research pace without affecting the earlier eras by much.
(Unrelated question: what does 'ICS' stand for?)
Infinite City Sprawl.
It means settling as many cities as possible, in as tight a pattern as possible, and keeping them small (size 4-5) to limit unhappiness; in the end it's impossible, graphically, to tell where one city ends and the next begins (hence the name). Build a Library and slot its Scientists, build a Colosseum, and with the right policies each city becomes a net positive (neutral happiness, gain science, gain money), especially if you go for the right Wonders (like the Forbidden Palace). You end up getting so much money that you can rush-buy whatever units you need, and you'll far out-science the AIs.
Many of the changes in the most recent patch drastically limited this strategy, but there are still a few advantages to making as many small cities as possible. There are quite a few threads in this forum discussing other ways to limit ICS. Basically, we dislike it because it's an easy way to win even on Emperor difficulty, and obviously the AI doesn't know how to do it so it feels even more like an exploit.