What method would you recommend (I assume it depends) to pass values from a form to to filter a set of pages returned as children of a node. There may be quite a few filer combinations so I thought a query string would get very long. The same with storing in memory, are there performance issues? I would have used cookies and JS in the past - there may be 1000's results and 5-6 filter with numerous combinations.

can you not serialise you filter to say f=000001 where that's filter 6 first option.. f=203010 and filter 1 option 2, filter 3 option 3, filter 5 option 1.. and remap.. that will keep your querystring short, and allow for bookmarking to return to the same filtering? not exactly human readable but heh..and depends on how many options per filter, though could go hex if it's more than 9 options per filter?

That's an interesting option. Currently there 5-6 filters but some can be compound creating a lot more to store. Are there performance and/or securtiy concerns around query strings?

Performance, not really. The mvc pipeline modelbinding is pretty good. Security wise, always depends what you use it for. But on a parsing/sanitizing side, .net/orms takes care of that for you in 99% of cases.