Is there a way to compare very short periods of activity in Oracle?
Directly query the DBA_HIST_ACTIVE_SESS_HISTORY view if you want a narrower view than an ASH or AWR can provide. Use something like this query:

select distinct user_id, session_id,session_serial#, sql_id, program, machine
from dba_hist_active_sess_history 
where sample_time between 
    to_date('20160819 1830','yyyymmdd hh24mi') and to_date('20160819 1835','yyyymmdd hh24mi')
and event = 'log file sync';