Particles Sprites taking too much to render

[Oscar Juárez <send.me(at)antropomorphia.com>]

first of all, sorry for the attatchment

the image i've attached is only a grid emitter, with 155 particles at 
the moment
a sprite made of an image sequence of 6 frames 256x256 px

aliasing of min 0 and max 2
treshold of 0.1
gaussian filter  3 3

on a dual xeon 2.8 with 2 gb ram
its taking 0:04:39.27
wich is very unnaceptable for 155 particles

if i connect the normal particle shape to the billboard
its taking 25 seconds... for my taste its still a bit longer
but much more acceptable that the sprite

is this normal... or did i do something terribly wrong

thanks in advance