SQL Server Spatial Query Performs Poorly at Low Zoom Levels - sql

Question

The following query performs very well at higher zoom levels (bounding box smaller than 2 degrees longitude by 0.5 degrees latitude, but degrades significantly as the bounding box gets larger. The table contains 7~8 million rows of text and location data stored as points in a geometry column.
I have tried different grid configurations, but the performance always degrades as the bounding box gets larger than something like #north=41.123029000000002, #east=-72.935406, #south=40.296503999999999, #west=-75.077740000000006
Any ideas? Thanks ~ Matt
I have included a table of the performance at different zoom levels.
declare #filter geometry
select #filter = GEOMETRY::STGeomFromText(
'LINESTRING(' + CONVERT(varchar,#west) + ' ' + CONVERT(varchar,#south) + ',' + CONVERT(varchar,#east) + ' ' + CONVERT(varchar,#north) + ')'
,4326
).STEnvelope();
select
x.Id
,x.[Timestamp]
,x.Location
,x.[Text]
from (
select top(1000)
t.Id
,t.[Timestamp]
,t.Location
,t.[Text]
from dbo.Table1 AS t with (nolock, index([SPIX_Table1_Location_HIGH]))
inner join containstable(
dbo.Table1
,[text]
, N'FORMSOF(INFLECTIONAL, "word1") | FORMSOF(INFLECTIONAL, "word2") | FORMSOF(INFLECTIONAL, "wordN")'
) as r on t.Id = r.[KEY]
where t.Location.Filter(#filter) = 1
AND t.[Hour] >= #HourId
order by r.[RANK] desc
) as x
order by x.[Timestamp] desc
option (maxdop 1)
Zoom Level 2 = #north=74.542308000000006,#east=94.21875,#south=-24.370607,#west=-180
Zoom Level 20 = #north=40.711250999999997,#east=-74.006050000000002,#south=40.710847000000001,#west=-74.007096000000004
This is the spatial index:
CREATE SPATIAL INDEX [SPIX_Table1_location_HIGH] ON [dbo].[Table1]
(
[location]
)USING GEOMETRY_GRID
WITH (
BOUNDING_BOX =(-180, -90, 180, 90), GRIDS =(LEVEL_1 = HIGH,LEVEL_2 = HIGH,LEVEL_3 = HIGH,LEVEL_4 = HIGH),
CELLS_PER_OBJECT = 16, FILLFACTOR = 70) ON [PRIMARY]
GO
Request Duration by Zoom Level (1000 random tests):
Level Duration0to5 Duration5to10 Duration10to15 Duration15to20 DurationGreaterThan20
2 0 0 0 0 26
3 0 0 0 0 42
4 0 0 0 0 57
5 0 0 0 0 60
6 0 0 0 0 54
7 0 0 0 1 65
8 0 2 5 6 34
9 0 3 7 10 6
10 5 23 25 14 1
11 13 26 18 3 0
12 17 31 7 0 0
13 48 11 0 0 0
14 48 6 0 0 0
15 47 1 0 0 0
16 57 0 0 0 0
17 48 8 0 0 0
18 44 3 0 0 0
19 63 5 0 0 0
20 47 3 0 0 0
ALL 437 122 62 34 345
Row counts and exec times for a typical query, w/out top(1000), forcing spatial index:
if (#level=2)
select #north=74.542308000000006,#east=94.21875,#south=-24.370607,#west=-180 --70,404 rows, 2 minutes w/ spatial
if (#level=3)
select #north=61.978465999999997,#east=-5.451886,#south=9.7052770000000006,#west=-142.56126 -- 57,911 rows, 1m22s w/ spatial
if (#level=4)
select #north=52.614061999999997,#east=-39.729228999999997,#south=26.230861999999998,#west=-108.283917 -- 45,636 rows, 1m23s w/ spatial
if (#level=5)
select #north=46.992624999999997,#east=-56.867901000000003,#south=33.775959999999998,#west=-91.145245000000003 -- 32,386 rows, 26s w/ spatial
if (#level=6)
select #north=43.934699999999999,#east=-65.437236999999996,#south=37.323439,#west=-82.575908999999996 -- 19,998 rows, 13s w/ spatial
if (#level=7)
select #north=42.343530999999999,#east=-69.721905000000007,#south=39.037540999999997,#west=-78.291240999999999 -- 11,256 rows, 13s w/ spatial
if (#level=8)
select #north=41.532438999999997,#east=-71.864238999999998,#south=39.879399999999997,#west=-76.148906999999994 -- 6,147 rows, 4s w/ spatial
if (#level=9)
select #north=41.123029000000002,#east=-72.935406,#south=40.296503999999999,#west=-75.077740000000006 -- 3,667 rows, 3s w/ spatial