Lower Frame rate for less polys ?

Hello everyone,

I am trying to create a very simple space shooter. I am importing premade 3D spaceship models and I have the following issues:

When I import spaceships made of 4k polyogons, I can can reach ~250k polygons before my frame rate drops to 350. However, when I import a single model of another spaceship made of 50k polygons, my frame rate drops to 100.

Here are the details.

Case 1 (350 fps):

 Main thread: 2.8 ms 
 Renderer: 0.1 ms
 Draw calls: 56
 Saved by batching: 0
 Tris: 247k
 Verts: 152k
 Used textures: 3 - 128 KB
 Render textures: 0
 VRAM usage : 9.7MB to 10MB (out of 0.97 GB)
 VBO total : 16 - 0.5 MB

Case 2 (100 fps):

 Main thread: 8.7 ms
 Renderer: 2.0 ms
 Draw calls: 84
 Saved by batching: 273
 Tris: 41.9k
 Verts: 53k
 Used textures: 6 - 470 KB
 Render textures: 0
 VRAM usage : 9.3MB to 12.9MB (out of 0.97 GB)
 VBO total : 346 - 3.1 MB

I also have another problem linked with this issue: My lateral movement command is:

 transform.Translate(Vector3.forward * LateralSpeed * Time.deltaTime, Space.World);

However, my spacesheep speed decreases with the framerate. Any idea why I have this?

Thanks!