Detta är en form av luckor-and-öar. Du kan använda en skillnad på radnummer för att få öarna:
select device_id, speed, count(*) as num_times
from (select t.*,
row_number() over (partition by device_id order by datetime) as seqnum,
row_number() over (partition by device_id, speed order by datetime) as seqnum_s
from t
) t
group by device_id, speed, (seqnum - seqnum_s);
Använd sedan ett annat lager med fönsterfunktioner för att få max:
select device_id, speed, num_times
from (select device_id, speed, count(*) as num_times,
row_number() over (partition by device_id order by count(*) desc) as seqnum
from (select t.*,
row_number() over (partition by device_id order by datetime) as seqnum,
row_number() over (partition by device_id, speed order by datetime) as seqnum_s
from t
) t
group by device_id, speed, (seqnum - seqnum_s)
) ds
where seqnum = 1;