In terms of how long the gates are down for I don't think it's as long as the OP may be suggesting. For example, taking Everton which is the one the OP said seemed busy, on the down fast line for a train to get greens all the way, the gates wouldn't need to be down until the train was passing through sandy (the double yellow indicating the crossing is still open to traffic is just north of sandy, slightly round the bend by one of the bridges). For a train on the down slow (which only has 3 aspect signalling) the gates wouldn't need to be down until the train had passed sandy north junction which is by the industrial estate a fair way north of the station.
Obviously I accept that there are 4 tracks to have clear for the gates to be open to traffic but with line speeds at Everton of 80 on the down slow, 125 on the fasts (and the slowest any scheduled train runs on the fast is 100 for the FCC fast peterbourghs) and 75 on the up slow it really dosnt take long for a train to get from the furthest restrictive signal to clearing the crossing itself. I would say that possibly it even keeps the traffic on roads moving more than some crossings by stations such as foxton which sees huge ques build up when stoppers go by as on the up road the signal needs to clear before the train reaches the platform and then take into account time for the train to stop, close doors again and start moving or on the down road it takes a while to get from the protecting signal into the platform. At least if the gates on the ECML crossings are constantly up and down it is letting a certain amount of traffic pass between trains.