you can define a function that downsamples in one dimension:

downsampleX = Total /@ Partition[#, 10] &;

Then call that twice to downsample in both directions:

Transpose[downsampleX[Transpose[downsampleX[sbt]]]];

Takes about 0.19 s on my PC

More elegant, but slightly slower:

downsampleX[downsampleX /@ sbt]

(Takes about 0.27 s)

you can define a function that downsamples in one dimension:

downsampleX = Total /@ Partition[#, 10] &;

Then call that once on your array and then on each row:

downsampleX /@ downsampleX[sbt];

Takes about 0.19 s on my PC