Tools such as Toad appeared 2 b very slow.
From mos I quote:
…The bug is fixed in patchset 184.108.40.206 and higher versions.
Release 220.127.116.11 and the patchset 18.104.22.168 are affected by this.
Since patching was not an option the workaround has been followed:
Mos note is offering create or replace view ALL_SYNONYMS.. and to set spfile value (_FIX_CONTROL).
Workaround has been implemented in Test and Preprod environments, no issues found with it and it showed improvement @ user side.