Hi Mario,
Deleting table entries is not a solution to solve performance issues.
As Dunning tables are tightly integrated with DFKKOP, so it will lead to System inconsistencies, if you go and delete the Dunning table entries.
Moreover, you will not be able to get the Dunning history for the delete items.
Have you identified whether its at the Proposal level or at the activity level.?
.
As next steps, check the following SAP notes, which talks about solving the performance issues for Dunning, depending on the SAP System version, that you are in
(a) Note 2129902
(b) Note 2105592
(c) Note 2000205
(d) Note 1997485
Check the Dunning events to see, if there is any customisation, which is having an impact on the performance. The important events to be checked are 1708,1712,0300,0302,0304,0311 etc
If the above options don't work, then raise an OSS with SAP and seek their advice on the same.
Hope it helps..
Thanks,
Amlan