I have some links between 15km and 25km with R532 in 5.8ghz and I use nstreme, polling and framer policy=best fit, the ping is of 8 - 12ms, but without nstreme, polling and framer policy, it is of 1 - 3ms
is it better not to use nstreme in these links?

It's ok to use nstreme but you'll need faster CPU then the one rb532 has in order to reduce ping time...

I ahve noticed this too

I use rb500 also

I do find that under load nstreame link will ahev lower ping than no nstream

I use nstream on ALL of my links, p2p or p2mp