Titan remove many vertices has poor performance - performance

Question

Improving performance when deleting multiple vertices?
I have a big graph in Titan (using rexster and cassandra) which I would like to be able to remove a potentially large portion of.
I want to delete everything a user "owns" which isn't in the provided list of IDs, and return the list of IDs of what they actually still "own" in the graph. I have tried these two approaches, both of which takes very long. I am testing with the worst-case scenario where list_of_ids_to_keep is empty([]).
//A naive list-based approach
def delete_mulitple(graph, user_id, list_of_ids_to_keep)
{
u = get_or_create_user(g, user_id)
user_things = u.outE('owns').inV().collect{it.id}
removees = user_things - list_of_ids_to_keep
g.v(*(removees)).remove() //This is the slow part of this approach
g.commit()
return (user_things - removees)
}
//Approach using Hash to potentially reduce memory footprint and O-notation, but is still taking a long time.
def delete_multiple_hash(graph, user_id, list_of_ids_to_keep)
{
def u = get_or_create_user(g, user_id)
def user_things = u.outE('owns').inV().collect{it.id}
def inUserGraph = [:]
def keepers = [:]
list_of_ids_to_keep.each { k_id ->
inUserGraph[k_id] = true
}
user_things.each {k_id ->
if (!inUserGraph[k_id]){
g.v(k_id).remove() //Again: the remove is the slow part
}
else{
keepers << k_id
}
}
g.commit()
return keepers
}
Is there a way to improve the performance of this using some sort of bulk operation, turning off some checks, doing an async query, or applying some other strategy?
I have already enabled "storage.batch-loading" which I though might improve performance. I measure the query times to be about 150 seconds, but then nothing is commited, so I believe it is timing out.
The layout of the graph is:
User (-owns->) A Thing (-contains->) A Description (can be shared by multiple "A Thing"s)
Numbers:
User -owns-> ~= 200K "A Thing"s. (g.v(user_id).outE('owns').count() ~= 200K)
A Thing -contains-> ~= 100 "A Description"s (g.v(a_thing_id).outE('contains').count() ~= 100)
A Description <-contains- ~= 100 "A Thing" (g.v(a_description_id).inE('contains').count ~= 100)