Phoenix query response time slow with secondary index

[#1]

Hi I'm using apache phoenix to query sql over hbase.

Table schema

CREATE TABLE TABLE_1 (
      SF_ID VARCHAR NOT NULL,
      ENTITY_ID VARCHAR NOT NULL,
      PRODUCT_SKU VARCHAR,
      CITY_NAME VARCHAR,
      SCREEN_NAME VARCHAR, 
      PRODUCT_LIST_VIEWS BIGINT,
      PRODUCT_LIST_CLICKS BIGINT,
      PRODUCT_LIST_CTR FLOAT,
      TIMESTAMP BIGINT NOT NULL,
      POS INTEGER NOT NULL,
      CONSTRAINT pk PRIMARY KEY (SF_ID, ENTITY_ID, TIMESTAMP, POS));

I have created secondary index as follows:-

CREATE INDEX GA_2 ON TABLE_1 (ENTITY_ID) INCLUDE (PRODUCT_LIST_VIEWS, PRODUCT_LIST_CLICKS, PRODUCT_LIST_CTR);

But the following query is taking around 1.5s to 2s when running on 500k rows.

select ENTITY_ID as "entityId", sum(PRODUCT_LIST_VIEWS) as "productViewSum", sum(PRODUCT_LIST_CLICKS) as "productClickSum", sum(PRODUCT_LIST_CTR) as "productCTRSum" from "TABLE_1" group by ENTITY_ID;

Explain plan is as following:-

CLIENT 1-CHUNK 0 ROWS 0 BYTES PARALLEL 1-WAY FULL SCAN OVER GA_2 SERVER AGGREGATE INTO ORDERED DISTINCT ROWS BY ["ENTITY_ID"]

Is there any way to optimize the response time of query?

[#2]

Update :-

Going by explain plan I created a salted table with 12 buckets.

Now the explain plan is following :-

+-------------------------------------------------------------------+----------+
|                               PLAN                                | EST_BYTE |
+-------------------------------------------------------------------+----------+
| CLIENT 12-CHUNK PARALLEL 12-WAY FULL SCAN OVER GA_3               | null     |
|     SERVER AGGREGATE INTO ORDERED DISTINCT ROWS BY ["ENTITY_ID"]  | null     |
| CLIENT MERGE SORT                                                 | null     |
+-------------------------------------------------------------------+----------+

But response time is still same.

Observed one more thing :-

If I don't use sum in my query response is fairly fast. 

For example 

select ENTITY_ID, SUM(PRODUCT_LIST_VIEWS) from GA_TABLE_2 where SF_ID = '1' group by ENTITY_ID;

this query took 631ms

but 

select ENTITY_ID from GA_TABLE_2 where SF_ID = '1' group by ENTITY_ID;

this took only 30 ms.

I'm new to apache phoenix. It would be really helpful if someone can help out.