If your sniffer freezes when you spoof, it means you have chosen the wrong arp targets. If it just lags you out, it's common for wireless clients on the sniffer, your router can't take it. If you're wireless all you have to do is just not sniff for too long. Don't sniff for longer then 5 seconds, simply stop after 5 seconds of sniffing, and keep going in that loop until you've gotten all your ips. This will decrease or completely remove the lag that you're getting from your session. If you're wired, issue will most likely be on your pc / internet and not on the sniffer.