A customer (Thanks Stefano!) alerted me on this issue during a workshop and I did some further investigation. Basic headline is:
Query on ALL_SYNONYMS is very slow in Oracle Database 126.96.36.199 compared to 11g.
The workaround for this 21324443: SLOW QUERY IN 12C ON ALL_SYNONYMS. is:
dbms_stats.gather_table_stats('SYS','OBJ$',estimate_percent =>100, method_opt => 'for columns flags size 1, spare3 size 254, type# size 254');
and I see that at least 8 other customers opened SRs hitting the same issue.
Does the workaround suit you in any way?
The bug had no progress since it was opened. If you are seeking progress I can only ask you to let support know
through the SR that you’d like to have the bug escalated.
Thanks for letting me know!
And please see also Bug 22113854 which is fixed now as the above one got closed partially as a duplicate of Bug 22113854. Fix is available on top of several Proactive BPs. Please check with Oracle Support.