In Europa Universalis/Common/government_reforms/05_government_reforms_natives.txt , after the "modifiers = {" at the very top, change "reform_progress_growth = -1.0" to "reform_progress_growth = -1.15".
You might have to tweak that number a bit, as I can't remember at which value it passes over to removing government progress all together. It's somewhere a bit over 1.
I just took a bit of time to test this modifier, and it doesn't seem to be working at all.
Using reform_progress_growth as a triggered modifier (I went quite overkill for the sake of testing, giving -2.0), and it still didn't show up at all, the native nation was still gaining 0.83 per month.
Even when I changed the government file and gave it "reform_progress_growth = -1.15", it still did nothing
Looking at it closely, it is meant to give a -100% modifier, which would prevent the natives from getting any reform progress until at least having a building to lessen the penalty, making the process much slower and is what seems to be the intended behavior.
But it doesn't work, and tweaking that value would still do nothing, so natives don't get that -100% modifier and end up reforming very fast.
This seems to be an actual bug, not a balancing issue, this modifier isn't working at all, and I think even for RT it was reported to not be working as well, it's the very same bug.
Your mod gave other conditions for reforms like using institutions and such, right?
This may have been what actually delayed the process, not the lack of reform progress.
For the sake of compatibility, I went for a different approach, I used a triggered modifier giving it a flat value with "monthly_reform_progress = -1", so they start the game with a net -0.16 monthly reform progress and need to boost the base value with buildings first before they can get any progress at all, and even then it'd be much slower.
