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

[hmidi slim]

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?