That probably makes sense: the multi-server mIRC requires much more overhead than a single-server mIRC. More coding is required, etc. to handle the complexity of multi-server connections.
As to the resource usage or freezing issue, it's hard to say. I've run mIRC 6.03 on several channels for a week or more at a time and haven't experience the problem you're reporting. It may depend on a combination of settings, features, etc.
Without a specific method to reproduce it, it's very difficult to trace I'm afraid.
