Easy, it is in assets/xml/gameinfo/civ4worldinfo.xml
For each world size, there is a part that looks like this:
<iResearchPercent>100</iResearchPercent>
It starts at 100 for duel-size maps, and goes up to 150 for huge maps. Techs cost this much % of the stated cost in techinfos, so at 150 a tech that costs 50 will actually cost 75. Just change it to whatever you want for the relevant worldsize(s). I've experimented with this a bit and at 300, tech progresses really slow even on a huge map. It takes a fairly long time to get out of the ancient age.
The other way to do it is to go and change the costs of each tech individually in civ4techinfos.xml ... or you can do both, I was thinking about ramping up the global cost of research and then lowering the cost of the ancient techs like agriculture etc, so that the ancient era would progress normally and then tech would start to bog down in the classical period.
In Civ4Erainfos.xml, there is also a line <iResearchPercent> for each era, starting at 100 for the ancient age, and finishing with, I think, 40 for the last era. I'm not exactly sure what this tag does, but I was worried it penalizes your research "income" based on the era you're in, in which case civs that went for an era-changing tech would get a penalty for researching techs from the previous era. Not sure the AI would use the correct strategy and fill out the tech for each era first. IF that's how that works ... maybe someone who knows could fill us in on that one.
