GPU alone 3x faster than with CPU?

[PantherX]

Out of curiosity, the next time you see it happen, can you please make a note of the PRCG, it would be any of these:
13422 (1644, 94, 1)
Project: 13422 (Run 1644, Clone 94, Gen 1)
project:13422 run:1495 clone:74 gen:2

[AveryAndrews]

I haven't taken notes of all of them (some real life disturbances), but I've got two 13421's, and the latest I've noted has been 13423 (8232,56,2). The base credits were all small (3190), leading to the hypthesis attached to my edited first msg.

UPDATE: And just picked up a new one, 13423 (7299, 7, 0), base=3190, estimated=14544, should be done in about 2 hrs.

[PantherX]

...Update/Answer: I think the key to the puzzle is given by bruce far below, which is that jobs are given to a range of cores that can get them done in time, so my conclusion is that the GTX 750 is near the top of one range but the bottom of a higher one, since the jobs it gets the Hi PPD on always have a rather small base credit, but it collects lots of bonus points, while the normal PPD ones have much larger base credits, so not so many bonus points.

If the assigned Project is a small one (low atom count), then it would run well on your GPU but run poorly on a high-end GPU.
If the assigned Project is a big one (high atom count), then it would run very poorly on your GPU but run properly on a high-end GPU.

Having said that, there's discussion about benchmarking various GPUs and then allocating them WUs that are ideal for their architecture in an automated manner. This is a big job and work is already underway on this. There's no ETA on when it will be live but watch this space