Performance problem on SWWCNTP0 tables

[Ritapac]

Hello everybody,
   my system is 4.7 (620 - SP lev. 0050).
Each activity on Workitems (log, execution,..) takes long long time, so the system performance are devastated; the performance trace (ST05) get a very long time on select in SWWCNTP0 table, but we have done all the activity to delete completed WI, reorg WF tables, upgrade kernel, and so on.
It seems that the select on this table is not optimized, but we haven't found any solution for this problem.
Do you have any idea about that?

Thanks in advance for your help.
Ritapac