Sorry, but I have a hard time accepting that making the game harder will slow the game down.
I agree with this. I haven't tried everything possible, but nothing so far has 'slowed the game down' by more than 2-3 more seconds per kill. That's hardly significant in terms of feeling 'longer' while actually playing.
I think if it were any longer, then enemies would start becoming Bullet Sponges / Albino Radscorpions. While that would certainly make it 'last longer', it'll just become an incredible hassle to kill enemies.
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For me, the level progression is actually quite good in vanilla. While I did take the liberty of reducing my XP gain in numerous ways, http://www.newvegasnexus.com/downloads/file.php?id=36411, and I pretty much set an example of how much I found it too fast here:
it's already fast enough so that 10 Deathclaws with the Well Rested bonus can earn you at least 10% of your next level at the higher levels.
If it were decreased just a bit, it could of worked out a lot better. Separating more enemies in different XP gain classes, I.E. Golden and Fire Geckos not being 55 XP fodder, may have worked as well as decreasing the overall XP gain of all enemies slightly. Even decreasing everything by 5 XP makes a lot of difference in the long run.
Decreasing it by 20, as shown in the mod I linked, makes leveling much more difficult via enemy kills. I pretty much "legitimately" earned all three ranks of Lord Death, i.e. with quest, area, and spawn mods, and I'm still not at level 30. :laugh:
It seems like I have more of a problem with the actual XP gains rather than the level XP requirements. I'll stop talking before I start going ranting on about quest XP and how silly some of those are. However, I do have to thank them for being so high or else I wouldn't even be close to level 25 on my character.