@advice on playing "higher then emperor" - well I really like "getting back up" from a bad start. On immortal it seems that if you start in a bad position you really are so far behind that you can never get back up. - Also it seems that things like using the perfect policies (read:
As far as turns: I play on marathon settings (Just because from experience otherwise militaristic units have become outdated before they reach the opponents). I do know that I am not really "victory minded" for the first part of the game, and even after that I first always try to "destroy" the runaway civ (through trade embargos / military / banning lux). After that I go for my own victory.
But my last game - here I played shoshone & amazon plus map. I started in the middle of the north eastern swamp. Playing "europe" just before I intended to go tall instead of wide (last map I tried going wide/liberty, but after founding 2 more towns I was landlocked and 3 civs were very angry at me for "settling wrongly").
Anyways I couldn't trade with anyone until my 3rd city (and only with suleiman after that, who was very far behind), and even lategame I could only trade with 3 civs. I won after 900 turns - but I was like 2 eras before anyone - in points I had more than all other 10 civs added together.
Well, in light of this post, your victory time is comparable to a turn 300 victory on standard. What this tells me is that your style of play simply leads you to a diplomatic victory. Others who play the game tend to find themselves finishing other victory conditions with their style. For me, I always finished domination victories first, because that is how I played until more recently.
900 turn diplomatic victories are not fast, even on marathon. Not that it is horribly slow, just that if you focused your efforts differently, you could easily win another victory condition sooner.
If you want to try and win other victory conditions, then you will have to adapt your play for another victory condition.