I think its changes with the person. For me its 40% concentration/consistency (my biggest problem), 5% tuning, 35% skill and 20% practice.
As for practice I noticed at monaco with only 10 laps practice i was going as fast as i could, yet at Fuji it took me 20 laps to get to my limit
I have the speed but i just cant do it lap in, lap out. In the race on sunday i set a 1.19.9, my fastest lap, the next lap i set a 1.11.2
Also some rather interesting results in pre season F1 testing:
1 Kimi Raikkonen (Fin) Lotus-Renault - 1:19.670
2 Paul di Resta (GB) Force India-Mercedes - 1:19.772
3 Nico Rosberg (Ger) Mercedes - 1:20.219*
4 Mark Webber (Aus) Red Bull-Renault - 1:20.496
5 Daniel Ricciardo (Aus) Toro Rosso-Ferrari - 1:20.496
6 Michael Schumacher (Ger) Mercedes - 1:20.794*
7 Kamui Kobayashi (Jpn) Sauber-Ferrari - 1:21.353
8 Jenson Button (GB) McLaren-Mercedes - 1:21.530
9 Felipe Massa (Brz) Ferrari - 1:22.815
10 Heikki Kovalainen (Fin) Caterham-Renault - 1:23.178
11 Pastor Maldonado (Ven) Williams-Renault - 1:23.371
12 Pedro de la Rosa (Spa) HRT-Cosworth - 1:23.676*