¿Es posible optimizar la consulta utilizando la cláusula EXISTS en lugar de IN con DISTINCT?

Prueba esta versión:

select contract_no AS c_no, cm_mac AS c_mc, min(tstamp) as time2, count(*) as aps
from devices d
where exists (select 1
              from devices d2
              where d2.contract_no = d.contract_no and
                    tstamp >= '2018-10-28 06:59:59' and
                    tstamp <= '2018-10-29 07:00:00'
              )
group by contract_no, cm_mac;

Quiere un índice en devices(contract-no, tstamp) .