Because the speedup effect on lvm cache was not significant (i could not measure it) i removed the lvm cache.

Surprisingly it seems that the cache was used a lot:

reox@kujira:~$ sudo lvremove kujira/lv_cache
Flushing cache for srv.
327680 blocks must still be flushed.
327635 blocks must still be flushed.
327582 blocks must still be flushed.
327530 blocks must still be flushed.
327469 blocks must still be flushed.
327349 blocks must still be flushed.
327231 blocks must still be flushed.
327124 blocks must still be flushed.
327004 blocks must still be flushed.
[...]