I have a K-3 with both the "FLUCARD PRO 16 GB O-FC1" and a "SanDisk Extreme" Class 10 80MB/s 32 GB.

The SanDisk stores images about twice as fast as my FluCard.

I've figured this with the following test:

• Erase the memory cards.
• Set the K-3 to manual exposure, 1/500th, Motor Drive H (9 shots/s).
• Hold down the shutter for as long as the camera takes pictures at high speed.
• Measure the time from pressing the shutter until the orange "saving" light goes off.

Results:

• With either card being the storage medium, I got to take 23 pictures.
• SanDisk takes about 18 seconds, FluCard takes about 38 seconds.

These results are consistent when repeated. And it makes no difference whether I place the card in slot 1 or 2.

Apart from these results, I don't have the impression that the card makes the camera "sluggish" in its general operation.

Mind you, though. I had also bought a Sony Class 10 32 GB card in 2014, supposedly even faster rated than the SanDisk. Yet, the Sony also was rather slow, within the range of the FluCard. I then found another batch of supposedly the same rated Sony card which was as fast in saving as the SanDisk. (More details in this Pentaxforums post of mine.) Meaning that at least Sony has good and bad batches of their cards without announcing it (they only print the reading speed on the label, but the critical one is the writing speed, which obviously varies drastically). Same may be true for the FluCard, i.e. there might be batches that are faster. But I doubt it unless someone claims the opposite :)

Despite the slowness, I am not much bothered by it. I just use the SanDisk in slot 1 and leave the Flucard in slot 2 for cases where I run out of memory (which is rare) or when I should need the Flucard's special features. Since even the K-3 can't write to both cards simultaneously, there is no speed advantage of using two cards anyway, i.e. even if I shoot RAW+, I still have both saved to the SanDisk card. Fills up more quickly this way, but I won't have a speed disadvantage by not using the slower second card at the same time.