Factors that affect framerate?

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The simulation world I recently build is very choppy when running. The world consists of:

• A ground plane with a tiled texture
• A hill consisting of 28 basic shapes (boxes and spheres) as well as texture attached
• A 4-wheeled robot model with a .dae mesh
• 3 point and 1 directional light sources
• 4 planes surrounding the ground plane that acts as a skybox (see this question: http://answers.gazebosim.org/question/6715/implementing-sky-texture/)

I'm running Gazebo 2.22 along with ROS Indigo on Ubuntu 14.04. My CPU is Intel Core 2 Duo, and my video card is Advanced Micro Devices, Inc. [AMD/ATI] RV516 [Radeon X1300/X1550 Series] with 256MB memory. The driver being used is simply "radeon". I've also gotten the following outputs:

1. Command:
find /dev -group video
Output:
/dev/fb0
/dev/dri/card0
/dev/dri/controlD64

2. Command:
glxinfo | grep -i vendor
Output
server glx vendor string: SGI
client glx vendor string: Mesa Project and SGI
OpenGL vendor string: X.Org R300 Project

3. Command:
glxinfo | grep render
Output
direct rendering: Yes
    GLX_MESA_multithread_makecurrent, GLX_MESA_query_renderer,
OpenGL renderer string: Gallium 0.4 on ATI RV515
    GL_MESA_ycbcr_texture, GL_NV_blend_square, GL_NV_conditional_render, 

4. Command: /usr/lib/nux/unity_support_test -p
Output: 
OpenGL vendor string:   X.Org R300 Project
OpenGL renderer string: Gallium 0.4 on ATI RV515
OpenGL version string:  2.1 Mesa 10.1.3
Not software rendered:    yes
Not blacklisted:          yes
GLX fbconfig:             yes
GLX texture from pixmap:  yes
GL npot or rect textures: yes
GL vertex program:        yes
GL fragment program:      yes
GL vertex buffer object:  yes
GL framebuffer object:    yes
GL version is 1.4+:       yes
Unity 3D supported:       yes

Is there anything I can do such as getting rid of unnecessary objects or adjusting the Real-Time/Sim-Time ratio to speed up the simulation in Gazebo?