I currently have a case running with Phase One regarding slider lag in C1 8.x on my main editing machine, and I have seen a similar report elsewhere, so it looks as if this latest version may realistically need some form of GPU acceleration as a minimum for optimum performance (though it's possible that the latest generation(s) of CPU might compensate for this to some extent.) The lag is noticeably worse than with v7.x, so it appears that something has changed in the v8 coding; Phase One themselves say "The update may effect [sic] speed overall as the new software is looking for more resources", though they also go on to say that they haven't observed the effect in a lower spec. machine.
My own system has a 6-core i7-980x Extreme Edition CPU running at the stock speed of 3.33GHz; not the latest, but by no means slow. However, the graphics card on this machine is an entry-level NVidia Quadro FX580, which has basic support for Open CL but does not meet the minimum spec. to enable this in C1, and I suspect this may be where the problem lies, though it has proven adequate until now. The slider lag on this machine is not so slow as to be unusable, but sufficiently so in some instances to be irritating.
Further testing on my laptop, which has a GeForce GT650M graphics card that does support OpenCL in C1, shows the response to be quite smooth with GPU acceleration enabled for the display, but the difference with display acceleration disabled is quite dramatic, with jerky and laggy response to slider adjustments. Both machines max out the CPU 100% during slider adjustment (in v8: less so in v7), but the experience with the laptop suggests this is probably not the primary bottleneck and that GPU performance is increasingly important.
I am considering upgrading the GPU in the main machine to either a Quadro K2200 or a K4200 (I reproduce artwork professionally, so ideally need the 30-bit output of the Quadros); Phase One's suggestion was that the K2200 should be sufficient, with the K4200 providing additional performance that may be better but possibly isn't needed. I'm awaiting their feedback on the latest results of the laptop tests, so will update with any further information or suggestions as received.
As an aside, I recently tested PhotoZoom Pro v6, which now supports Open CL, and the processing speed on my editing machine with Open CL enabled was substantially slower than with it disabled - it appears that if the Open CL performance is insufficient, the overhead involved in shuffling data between the CPU and the GPU actually causes a drop in performance. That may be where your additional delays are occurring when you have Open CL enabled.
