Slow
Slow, and I HAVE experienced a couple of rejected deposits recently, although none of the cock-ups where the money goes from Neteller, but fails to make it to the casino. Logins to Neteller have been very slow too, but I have eventually managed to get in.
It seems it's mostly the website, but occasionally there is a timeout, and a casino rejects a deposit because Neteller's server has not responded.
I always check my Neteller before trying again when this happens, because I remember the bad old days of money leaving Neteller and ending up in limbo, and simply trying again only makes things worse.