On the minor arcana

[Spaced]

The thoth deck matches these concepts to astrology with Mercury in Sagittarius which I think reflects the same ideas.

what do you think about the rainbow in that card?

I guess it means that 'Swiftness' is able to encompass the whole spectrum (which is 7 colors of the rainbow - but the full set is an octave of '8').

I guess the main image also goes in 8 directions (of the compass).  

So this card/experience is not limited in it's range.

But the geometry is pointing towards an octohedron (a d8 from dungeons & dragons)

According to Crowley the rainbow is another symbol related to Sagittarius. The hebrew name for Sag is קשת, which means rainbow. The rainbow is also present in the Art (Temperence/The Alchemist) in the Thoth deck, which is also associated with Sagittarius. The bottom 3 paths of the tree of life, the ones leading from Malkuth, are represented by ת (the Universe) ש (the Aeon/Judgement) and ק (the Moon) which spell out קשת, Rainbow. Saggitarius (the Art) is the path that transcends the lower paths leading from Yesod to Tiphareth. The rainbow is the bow that propels the arrow onward.

I think that the reasoning for the electric rays on this card is related to the idea of the fire being sublimated into rays of energy and and light (this is the only card in the suit that doesn't portray flames). On the micro level this includes concepts like brainwaves and speech, on the macro scale it includes light, electricity, etc. The rays are seen creating and sustaining the geometric shapes representing the energetic basis of matter.

So in essence yeah, the card deals with limitless and full spectrum energies.