more_fair.py

import bpy

# # slow
# for x in range(-25, 25):
#     for y in range(-25, 25):
#         bpy.ops.mesh.primitive_cube_add(radius=0.2, location=(x, y, 0))
# 

# faster, much faster
import bmesh
ctx = bpy.context
for x in range(-25, 25):
    for y in range(-25, 25):
        bm = bmesh.new()
        bmesh.ops.create_cube(bm, size=0.2)
        me = bpy.data.meshes.new('Cube')
        bm.to_mesh(me)
        bm.free()  # free and prevent further access
        obj = bpy.data.objects.new('Cube', me)
        obj.location = (x, y, 0)
        ctx.scene.objects.link(obj) 

possibly_faster_still.py

# faster, much faster
import bpy
import bmesh

ctx = bpy.context
objs = []
add = objs.append

for x in range(-25, 25):
    for y in range(-25, 25):
        bm = bmesh.new()
        bmesh.ops.create_cube(bm, size=0.2)
        me = bpy.data.meshes.new('Cube')
        bm.to_mesh(me)
        bm.free()  # free and prevent further access
        obj = bpy.data.objects.new('Cube', me)
        obj.location = (x, y, 0)
        add(obj)

for obj in objs:
    ctx.scene.objects.link(obj)