Follow up:
Now I tried a Plugin-Performance-Measurement:
While I scan my site with "P3 (Plugin Performance Profiler)", i get:
A fatal error occurred during profiling: Allowed memory size of 134217728 bytes exhausted (tried to allocate 41 bytes) in file /is/.../wp-content/plugins/p3-profiler/classes/class.p3-profiler.php on line 297
When I deactivate WPPA+, the performance-scan finishes without the memory-error.
128 MB memory seem to be a good value, and I think, a gallery should run with that. Maybe this is a hint, why WPPA+ slows my website so much?!
Any tipps would be nice!