Your use of 'dimensions' puzzles me. Do you live in a world of 3 dimensions + time for a 4th? Your logic about relating dimensions to speed (# squares moved per tempi) is confusing you. Does an airplane that moves faster than you operate in a different number of directions? Chess has two dimensions, + time; then you have as stated before, the up-down dimension, and left-right dimension. Diagonal is not a new dimension. A piece can stay on the same square too, if a different piece is moved, so that's the time dimension.
Good luck confusing yourself more. You're quite caught up in maintaining that your 1.5 d idea is correct. I can see your argument, sort of, but I see it in the light of you conflating the usage of the concept of dimensions. :)
I mean I understand what you're saying. But it seems that it requires some assumptions that just aren't true. You have to really assume that each piece is only allowed to move a finite distance in a tempi. But this is obviously not true under the rules of chess. If this was the case, then the piece obviously wouldn't be able to move diagonally within the rules of 2-D movement, but all this seems irrelevant since the underlying assumption of fixed distance movement isn't the case.
Am I making sense or just rambling?
Perfect sense, and i see what your getting at. Each piece is allowed only finite movement per tempi. As I said, take the Bishop. It has a range of 'speed' of between 1 and 7 'squares per tempi'. In any given instance, the maximum number of squares it could move is seven, assuming it were at the corner of the board and had a clear diagonal ahead of it. If a chess board were 20x20 squares, a bishop would have a maximum speed of 20 squares per tempi, depending on its starting location. The finite distance, and therefore speed, is a consequence of the limited space on a board. I stress though, that this is not actual speed, e.g. the speed that you could physically move a piece with your hand, but tempi-based speed, which exists only in the abstract.