Query lenta:trova la differenza tra i valori in base al minimo e al massimo su un'altra colonna per ciascun gruppo

Se ho capito bene, vuoi la differenza tra le due righe più recenti per ogni fix_id dove type = 'avg' .

In tal caso, suggerirei variabili e aggregazione condizionale:

select fix_id,
       max(case when rn = 1 then odds end) as odds,
       max(case when rn = 1 then market end) as market,
       max(case when rn = 1 then away end) as away,
       sum(case when rn = 1 then odds when rn = 2 then - odds end) as diff,
       max(type) as type
from (select ao.*,
             (@rn := if(@f = fix_id, @rn + 1,
                        if(@fn := fix_id, 1, 1)
                       )
             ) as rn
      from (select ao.*
            from average_odds ao
            where type = 'avg'
            order by ao.fix_id, ao.updated desc
           ) ao cross join
           (select @f := -1, @rn := 0) params
     ) ao
where rn <= 2
group by fix_id;