Use an old gsdx version. 1.07 I think.
You could try using the software mode in GSdx as it also fixes some big screen bugs for this game and you have a machine to work it fast enough. If still not fast enough (or just dont like it) then as Blackclaw said the last fast working GSdx in hardware mode for this game was GSdx 0.1.7
yeah, but in the forest it happens again. the image all broken and fps will drop again. in the battle scene, the fps will normal though.
i use 0.1.7 (i forgot too), and i can easily get 60 fps in first town.
i think there are no resolutions around the forest issue, except you run it at software mode. which ridiculously slow.
yeah, i hope bositman, krakatos, rama, or gabest can tell us (you and me, noobs) more about this.
It's strange with my poor E7200 @3.6Ghz and my old 8800GTS I have around 45 fps in harware mode in the forest (the bug is a pain so I choose to play it on my ps2 until solved) so you should be able to achieve at least the same kind of speed.
What's there to speculate about? The scene is severely bugged in GSdx hw modes, causing the visual and speed problems.
As no one knows what's happening there, no fix exists.
I've read however that only this forest level has the bug.