postgresql - How to fix performance problem in PostGIS ST_Intersects?

I'm a newbie in postgis and I have a problem in query performance.

This my query:

SELECT DISTINCT ON (userid) userid ,ST_AsText(position), timestamp  

FROM table1 
WHERE ST_Intersects ( ST_GeomFromText('a multiypolygon geom goes here',4326),position) 
ORDER BY userid, timestamp desc

and the problem is my multipolygon include VERY large polygons (600 pages long in word doc!) and it took more than 2 hours to execute!

Is there a way to optimize my query or use another way?

Please your help is greatly appreciated!