WorldEdit Help

I've been using WE for a few years, and I reinstalled it again today. I enabled, "//fast", then, I tried to make floating snow for a roof of my modern home. When I did, "//set snow:1" it didn't show up instantly, because of "//fast". When you do, "//fast", it requires you to leave the game every single time you make floating snow or something else like that nature. I remember back on 1.6.4 when I first build a modern home and uploaded it on here (the one I'm creating now, making it way more modern), I didn't have to leave the server every single time. May I also say that I'm on singleplayer. Can someone please help? I'm using 1.7.10 with Optifine and WE 1.7.10. I couldn't find a 1.8+ WE, that's why I'm on 1.7.10. Thanks!