take a lot of time in process between NXT and BTC blockchian
This depends on the time between Nxt blocks, the time between BTC blocks and the status of the servers.
Development servers require ten confirmations in the Nxt blockchain before accepting a Nxt transaction (a deposit address request, or a mgwBTC withdrawal). Past those confirmations, if the three servers are up they will catch and queue the request. Servers process periodically the withdrawals in batch and sign the BTC transactions, that will be included in the BTC blockchain also following its block times.
So the processing times in MGW are variable, although if nothing causes an extra delay these times will oscillate between 45 and 90 minutes (time between requesting the withdrawal and seeing the BTC transaction pop up in your wallet). But if one of the development servers is down, for instance, or requires maintenance, the total processing time will grow as much as the downtime/maintenance time for the server.
As has been said before, we expect to minimize down times (thus speeding up processing times) in the production servers. These are already working and they will be open to users as soon as stress test cases are finished and a new GUI is available.