I don't think you can adjust speed in fractions (AFAIK, anyway), but you can reduce the speed increase for each type of engine as well as increase the research times.
There are two files you would need to edit for each component - TechTree.xml and GC2Types.xml.
The best engine is the Hyper Warp Drive Mk. III, and I'll use that as the example.
Since each newer engine would naturally give a speed boost, and they only increase by 1 (at most - many are just a smaller size/sizemod/cost with the same speed as the previous) with each new type, it is rather pointless to try and reduce the actual speed.
Instead, increasing the 'Size' would make it so less of them could be fitted on any given hull, and an increased 'Sizemod' would make it even harder on the larger hulls.
Increasing the 'Cost' in TechTree.xml would make them take a longer time to research.
TechTree.xml
[Propulsion ID="HyperWarp III"]
[DisplayName]HyperWarp III[/DisplayName]
[Cost]3200[/Cost] (changing the cost to 6400 would double the research time required)
[Description]Superior form of HyperWarp propulsion system.[/Description]
[Details]Who would have thought that HyperWarp could get even better? We sure didn't, but here it is![/Details]
[Requires]HyperWarp II[/Requires]
[Model]hyper320[/Model]
GC2Types.xml
[Drive Name="Hyperwarp2"]
[DisplayName]Hyper Warp Drive Mk. III[/DisplayName]
[Cost]10[/Cost]
[Size]5[/Size][SizeMod]5[/SizeMod][Speed]6[/Speed]
[Model]Hyperwarp2[/Model]
[Category]Engine[/Category]
[Tech_Requirement]HyperWarp III[/Tech_Requirement]
[/Drive]
You can do the same type of thing with the hull techs.
And, there is a mod in the Library that will allow you to adjust the overall tech speed by increasing or decreasing the research times. I have not used it, but I have noticed it.
WWW Link(The 18x18 sized galaxy is the largest, by the way.)