Replistor reserves an area of memory called the kernel cache for catching all file system changes - the amount of memory requires some configuration to set correctly (see manual - the procedure is in there) - by default it is 5% of physical memory but this can be changed. You need to bear in mind that this memory is reserved so is no longer available to your applications.

It could also be that replication may be causing some contention for disk resources if the kernel cache is inadequately sized and is overflowing to disk. I would suggest firstly ensuring the kernel cache has been sized correctly,