You can't blame GI to have such long calculations times without acutally knowing the technique to calculate the gi.

By just turning this two letter (GI), u r switching on the most complex mathematical algorithms in ur 3d software. The techniques like photon mapping, irradiace mapping, QMC sampling, BSDF, BRDF (Bi-Directional Reflectance Distribution Function, Bi-Directional Scattering Distribution Function)etc. have very complex algorithms based on complex mathematical formulas, etc. the renderer must take in account of every light in the scene,every shadows, every materials, whether they r specular, glossy, nonglossy, transparent, refractive etc and the has to trace every rays from the light or from the camera to the objects till the limit to finally shade each sample pixel. So this can take very long time.

and I think, this is why Calculating gi is very slow.

If ur getting horrible results, u should learn ur renderer well, then u have to tweak the GI settings

Normally, U can use a low quality setting to preview the gi lighting & if the lighting is OK, then switch to the high quality setting for the final render, so the trick is in controlling the GI

If GI is very expensive for u, then there r other techniques like Ambient Occlusion, Radiosity etc to calculate the Final Illumination.

Just blame ur Pentium 4 or AMD 64. 'cause still, even these processing power is not enough to calculate the GI solution in just 10 or 20 seconds for a normal desktop user, I think.

And finally, the antialiasing - (Not a part of GI)- takes time to sample every edges of the scene objects that can also increase the final render time.