I first played the game on my Windows box, then I decided I'd give it a try on my Linux box, and it works quite well once I resolved my missing OpenGL extensions problem.

However, I noticed that everything in the game is slower than when I was playing on Windows: character moves slower, enemies move slower, etc. It's actually quite disturbing, and on the verge of frustrating as well, particularly after playing it at normal speed.

Anyone has an idea for fixing that?

Below some informations on my rig (hardware wise, it is nearly identical to my Windows box, with small improvements here and there), extracted from GOG System Report.

Distribution: SolydXK
Kernel: 3.14-1-amd64
Architecture: x86_64
Processor Name: Intel(R) Core(TM) i5-2500K CPU @ 3.30GHz
Processor Speed: 1803.785
Processor Cores: 4
Model: NVIDIA Corporation G92 [GeForce 9800 GT] (rev a2)
Driver Module:

Tests: direct rendering: Yes
Display:

Screen 0: minimum 320 x 200, current 1920 x 1200, maximum 8192 x 8192
DVI-I-1 connected primary 1920x1200+0+0 (normal left inverted right x axis y axis) 518mm x 324mm
1920x1200 60.0*+
1600x1200 60.0
1280x1024 75.0 60.0
1280x960 60.0
1152x864 75.0
1024x768 75.1 70.1 60.0
832x624 74.6
800x600 72.2 75.0 60.3 56.2
640x480 75.0 72.8 66.7 60.0
720x400 70.1
DVI-I-2 disconnected (normal left inverted right x axis y axis)

Memory Total: 8180956 kB
Swap Total: 12280664 kB
Swap Free: 10768040 kB

You claimed to have installed dxtn.
That means you use Open driver, as proprietary from Nvidia already includes that.

The open driver for nvidia is a hit-or-miss. 80/92a chips (8800/9800) should be good enough supported by nouveau, but I suggest to install nvidia proprietary (legacy) driver if you want full 3d performance, or:
- buy amd (but not the very very recent gpus)
- buy newer nvidia (from kepler and up)
- buy top-tier intel (other provide bad performance)