ropebih, you should try with latest wireless-test package as that includes improvements discussed here:
http://forum.mikrotik.com/t/nstreme-test-package/24288/1
this could help latency issue.
Anyway you should be ready for latencies higher than with regular non-nstreme mode and potential jitter. Causes for this can be multiple, but this can be considered the price to pay for increased throughput.
As to the explanation macgaiver gave - it is only partially true: nstreme does not intentionally delay any packets (waiting for more packets to send in one frame). Nstreme starts to aggregate packets when queue of packets waiting to be sent starts to form - that is, when rate at which packets arrive exceeds the capacity of link. This way no latency is added by the sending party. Still aggregating packets actually does increase latency, but this is caused by receive processing - whole big frame must get received (e.g. to check CRC) before further processing as separate packets, and in case aggregated frame is lost, it obviously takes more time to resend.