Oracle DB occasionally slows down when "Setting Resource Manager plan DEFAULT_MAINTENANCE_PLAN"
Seems like there are some Database Resource Manager (DRM) constraints put on user sessions when the database switches to maintenance plan via Scheduler window each night (as evident by your log portion).

There are at least two predefined DRM plans when a database is created.


DEFAULT_PLAN
DEFAULT_MAINTENANCE_PLAN


(See the complete list.)

When a window activates, it can switch the current resource plan to one specified in the window. You can query ALL_SCHEDULER_WINDOWS view to see which plan each window is assigned.

You can also switch between plans in Enterprise Manager or manually changing RESOURCE_MANAGER_PLAN initialization parameter value.

You can use Enterprise Manager or DBA_RSRC_* views to see


which plans are active in database – DBA_RSRC_PLANS
which resource consumer groups are defined – DBA_RSRC_CONSUMER_GROUPS
which group mappings are defined – DBA_RSRC_GROUP_MAPPINGS
which resource plan directives are defined – DBA_RSRC_PLAN_DIRECTIVES
and so on


Resource plan directives is what defines resource usage by the database sessions. The sessions map to consumer groups via group mappings.

When your database switches to another plan (using window or manually), the different set of directives is enabled which controls the resource usage by sessions. Thus when your database switches from DEFAULT_PLAN to DEFAULT_MAINTENANCE_PLAN and vice versa, your user sessions notice the difference – their performance dynamically changes according to the resource directives defined in the enabled plan.

For more details on implementing and using resource plans in DRM, consult the Managing Resources with Oracle Database Resource Manager chapter in Database Administrator's Guide.

UPDATE

You can check which directives are defined for plans in your database.

select plan, group_or_subplan, mgmt_p1, mgmt_p2, mgmt_p3
from DBA_RSRC_PLAN_DIRECTIVES
where plan like ('DEFAULT_%')
order by plan, mgmt_p1 desc, mgmt_p2 desc, mgmt_p3 desc;

PLAN                        GROUP_OR_SUBPLAN    MGMT_P1 MGMT_P2 MGMT_P3
------------------------    ----------------    ------- ------- -------
DEFAULT_MAINTENANCE_PLAN    SYS_GROUP                75       0       0
DEFAULT_MAINTENANCE_PLAN    OTHER_GROUPS              0      70       0
DEFAULT_MAINTENANCE_PLAN    ORA$AUTOTASK_SUB_PLAN     0      25       0
DEFAULT_MAINTENANCE_PLAN    ORA$DIAGNOSTICS           0       5       0
DEFAULT_PLAN                SYS_GROUP                75       0       0
DEFAULT_PLAN                OTHER_GROUPS              0      90       0
DEFAULT_PLAN                ORA$AUTOTASK_SUB_PLAN     0       5       0
DEFAULT_PLAN                ORA$DIAGNOSTICS           0       5       0


Notice how resources are allocated between different consumer groups. Here OTHER_GROUPS and ORA$AUTOTASK_SUB_PLAN are allowed to use 70% and 25% of CPU respectively in DEFAULT_MAINTENANCE_PLAN plan, while in DEFAULT_PLAN they are allowed 90% and 5% respectively.

It means that these directives might be one of the reasons your users' sessions run slower when maintenance windows are open, because users are allocated less resources than what they are during work hours.