Low disk performance?

Hello everybody,
having this SQL query:

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select
variable_id,float_value,ts,good_through,interval,datetime_value,string_value,int_value,blob_value,history_value_type
from "records_437954e9-e048-43de-bde3-057658966a9f" where variable_id
in (22727) and (ts >= '2010-10-02 11:19:55' or good_through >=
'2010-10-02 11:19:55') and (ts <= '2010-10-14 11:19:55' or
good_through <= '2010-10-14 11:19:55')
union all
select
variable_id,float_value,ts,good_through,interval,datetime_value,string_value,int_value,blob_value,history_value_type
from "records_1d115712-e943-4ae3-bb14-b56a95796111" where variable_id
in (24052) and (ts >= '2010-10-02 11:19:55' or good_through >=
'2010-10-02 11:19:55') and (ts <= '2010-10-14 11:19:55' or
good_through <= '2010-10-14 11:19:55')
order by ts
limit 2501 offset 0

---------------

and these two results:

1st run:
http://explain.depesz.com/s/1lT

2nd run:
http://explain.depesz.com/s/bhA

is there anything I can do about the speed? Only buying faster
hard-disk seems to me as the solution... Am I right?

Thank you in advance
  Martin