how much postgres can scale up?

I have a function in pgsql language, this function do some select to some
tables for verify some conditions and then do one insert to a table with NO
index. Update are not performed in the function

When 1 client connected postgres do 180 execution per second
With 2 clients connected postgres do 110 execution per second
With 3 clients connected postgres do 90 execution per second

Finally with 6 connected clients postgres do 60 executions per second
(totally 360 executions per second)

While testing, I monitor disk, memory and CPU and not found any overload.

I know that with this information you can figure out somethigns, but in
normal conditions, Is normal the degradation of performance per connection
when connections are incremented?
Or should I spect 180 in the first and something similar in the second
connection? Maybe 170?


The server is a dual xeon quad core with 16 GB of ram and a very fast
storage
The OS is a windows 2008 R2 x64

Thanks

Anibal