[Rspamd-Users] enable slow timer delay - HOW?

[Alain de Longue]

After upgrading my email server to Debian stretch, I now see many of the followingin rspamd.log
2021-03-12 13:54:15 #30968(rspamd_proxy) <c18322>; proxy; rspamd_symcache_finalize_item: slow rule: RBL_VIRUSFREE_UNKNOWN(265): 531.45 ms; enable slow timer delay
2021-03-12 13:54:15 #30968(rspamd_proxy) <c18322>; proxy; rspamd_symcache_finalize_item: slow rule: RBL_NIXSPAM(252): 1115.04 ms; enable slow timer delay
2021-03-12 13:55:54 #30968(rspamd_proxy) <c640d9>; proxy; rspamd_symcache_finalize_item: slow rule: ASN_CHECK(225): 560.56 ms; enable slow timer delay
2021-03-12 13:55:55 #30968(rspamd_proxy) <c640d9>; proxy; rspamd_symcache_finalize_item: slow rule: MAILSPIKE(268): 650.97 ms; enable slow timer delay
2021-03-12 13:55:56 #30968(rspamd_proxy) <c640d9>; proxy; rspamd_symcache_finalize_item: slow rule: RBL_VIRUSFREE_UNKNOWN(265): 360.94 ms; enable slow timer delay

This did not happen when the server was running Debian jessie.
I cannot find in the rspamd documentation how to "enable slow timer delay".
Any help is appreciated.
a.s.

"I am a part of all that I have met." Tennyson.