How can I mine same time on CPU and GPUs?
I'd suggest you run the CPU mining with -t 3, to use just 3 cores, leaving one free for GPU miner duties (relevant if that CPU coin miner is already optimized for AES-NI). In any case, you can play with the number of threads for CPU mining all the way up to 7, just not the default (which uses all 8 threads).
I usually go into the task manager details and set the affinity of the GPU miner to be on my 1st core/thread (or the 1st two), and set the affinity of the CPU miner to the other ones, thus isolating one from the other.
(if you're not using windows, please ignore

)
~ Myagui
This makes me think that ccminer just has difficulties with cross-threading on the cpu. If work A is on core 0 and work B is on core 1, but you're also CPU-mining on those cores, ccminer will have to pause a bit, and he doesn't like that

Also, Intel knows how to deal with threads, so this might be why a Celeron is a winner for ccminer.
So yeah, try setting affinity to lock the processes on cores.
Please report back if that did or didn't do it

If it did it, it's something I will implement in my ccminer gui. Should you set the affinity per ccminer, like ccminer30 on core0, ccminer50 on core1, etc... or does it matter at all? I see no real difference when I put them in one thread on my system (i5-3470)
But before you ask, I won't do a gui for minerd
I have 8 logical threads on CPU - I've set 4 to CPU miner and another 4 to GPU miners - works fine
However, when I tried to assign only 2 logical threads to GPUs then I was loosing hashrates, 10% drop. I think it might be related to fact that I have 2 GPU cards, hence 1 physical core per 1 GPU card (i7-4770k has 4 physical cores and 8 logical)