média de execução no mysql

SELECT t.*,
       (select avg(speed) from tbl tt where tt.timestamp <= t.timestamp) as avg
FROM tbl t;

select t.*, (sum_speed / cnt) as running_avg_speed
from (select t.*, (@rn := @rn + 1) as cnt, (@s := @s + speed) as sum_speed
      from tbl t cross join
           (select @rn := 0, @s := 0) params
      order by timestamp
     ) t;