Let's agree that, no matter what goes on inside the mixer, coins go in and later coins come out; what achieves anonymity is the inability to connect inputs with outputs. Therefore small batches and a small latency window will reduce anonymity. Consider the case of a coin sent to the mixer and a coin that comes out 36h later, or a couple of weeks later - could they be connected? Seems better not to set short time limits.

Also do not forget that an attacker will be spamming the server with their transactions, both for DoS and to fill up any small batches. Of course for each "real" input there will be a probability that the mixer adds a dummy input and output. (And of course all communication with the server is encrypted and dummy traffic generated.)