how to increase frequency

truncating 3 bits(instead of explicit division):

for 8 values average you need to divide the sum by 8 i.e. 2^3 so all you have to do is discard the 3 LSBs from sum4.

using mult instead of divide(since dividers can be slower than multipliers):

to divide by 9, final sum = sum4/9 = sum4 * 57/512

i.e. multiply sum4 by 57 then discard 9 bits off final result.

The value 57 is derived from 512/9

you can and should use more bits for more accuracy:

final sum = sum4 * 3641/32768 then discard 15 bits