How can a large collection be filtered on the server side by a numeric field? I found documentation for numberFilter, but not serverNumberFilter.
you can create own serverNumberFilter:
webix.ui.datafilter.serverNumberFilter = webix.extend({
$server: true
}, webix.ui.datafilter.numberFilter);
or even use serverFilter as serverNumberFilter.
on the server side you will need to parse filter value to get compare operator and actual value.