What about a combination. Create a temp computed for the subscribeables you need to read but do not want to subscribe to. Changing them would update the temp computed but that could be a cheap operation. Your real computed reads the tempComputed with peek accessing the currently cached value.
// this one is updated
// if any of the subscribeables used in readSomeMoreObservables changes
// but that is hopefully cheap
var tempComputed = ko.computed(function() {
readSomeMoreObservables();
});
var computed = ko.computed(function() {
readSomeObservables(); //<-- if these change, notify computed
// do not update on readSomeMoreObservables
tempComputed.peek();
});