Removing Outliers Grouping by Minute
It seems there is no alternative to solve the problem without using a sub query, what is a performance constraint in my case, so I decide move to another approach.

I gave up about the trimmed means (thanks @Linas) and move to a percentile_disc indicator. Once all I wanted was to give a measure of latency for each request I did a simple query as follow:

select 
  mdate,
  mtime,
  percentile_disc(0.997) withing group (order by latency) as "response" 
from mtable
where url like '%MyPartialUrl% 
group by mdate,mtime
order by mdate,mtime


As this way I grab the biggest latency took to answer a request for 99.7% of all users. In fact I am grabbing several percentiles following six sigma approach to show how much slowness per million requests we are getting on production.

Query took no more than 8 seconds to be executed, using the trimmed means before it was around 90 minutes.

Thanks all.
Regards