P: Slow performance on Xeon CPUs

That makes sense. On startup LR determines the available processor cores and distributes the workload using a muli-processing algorithm. With 6 or more cores the algorithm fails and LR's performance drops. If this is the case then restricting the available cores on systems with 6 or more core processors should improve performance. I suggest also testing with hyperthreading both enabled and disabled in the system BIOS.

Puget Systems did similar testing of LR6, but they didn't run any benchmarks for Develop module control performance (Tone, Adjustment Brush, etc.).
https://www.pugetsystems.com/labs/articles/Adobe-Lightroom-CC-6-Multi-Core-Performance-649/

Also interesting: I have made all tested with photos of my D5100 (RAW 15 MB). Now, I tested with photos of my D800E (RAW 35 MB). The problem remain exact the same. It is interesting, that the problem is not aggravated with bigger images. Exact the same.

I think that this excludes memory problems.

But I find here in the forum also people using 6 and more cores with problems! Examples: https://feedback.photoshop.com/photoshop_family/topics/lightroom-2015-6-extremely-slow

But yes, I think it's a combination of LR-PC-Configuration.

I exclude the graphics card, because I have already tested with two different cards in the same PC. And I exclude drivers, because I have updated all system components.

Now I look for people who also have a XEON E5-1650 0 3.2Ghz and be able to report their experiences.

Your English is quite sufficient!

Large numbers of people use LR with 4 cores (8 virtual processors).  There have been a few reports on this forum of people successfully using 6, 8, and 10 cores (12, 16, and 20 virtual processors).   So there must be something particular about your configuration or hardware.

Unfortunaly my englisch is very bad.

a)
I would say: with 4 cores LR is faster than with 6 cores:

4 cores: faster and smoother
6 cores: slower and not so smooth

b)
Yes, LR does get slower over time and then become unusable:
with 4 cores: over more time
with 6 cores: over less time

No, when I do nothing, then no problem.

I believe you indicated that with 6 cores, LR would slow down more and more after 3 photos, soon becoming unusable.  With 2 or 3 cores, LR clearly will run slower; but does it get slower over time and then become unusable, or does it just stay slow without getting worse?

a) no memory usage not increase dramatically. Memory usage stay around 30 - 50 %. Never more.

b) with your tip I tried different number of cores. With 1 core: slower, with 2 cores slower, with 3 and with 4 cores faster. The problem is not resolved with 4 cores, but seams not so dramatically. The brush is smoother and a little faster than with all 6 cores. On images with not so much local corrections, the difference (4 cores to 6 cores) is bigger (4 cores faster than 6 cores). With images with many corrections, the difference is not so big.

Seams that core support is the crucial point in LR.

No. The use of "cmd.exe" is required with the "start" command, probably because of something funky LR does on its start up.

I don't think using the Task Manager to set affinity of a running LR process would be as good a test. It's possible that LR makes some decisions about how many processors to use early in its startup sequence, and setting affinity with the Task Manager would be too late.

John, are you saying using Windows Task Manger's 'Set Affinity' does not work with LR?

Note that the seemingly superfluous use of "cmd.exe" appears to be necessary due to the way that LR starts itself.