Why the creation of spatial indexes decrease the performance of a query?

[hmidi slim <hmidi.slim2@xxxxxxxxx>]

Hi,

I'm using postgresql 9.6.4 on linux and I'm trying to execute this query on a table contains 51000 rows.

and I got this query plan:

​​

Creation of index:
​I get this queryPlan:

​Why the execution time increases when creating a spatial index on a table containing a large amount of data?