Boss Tricks: Parasite Queen

Notes:

Quickly beating the Parasite Queen is almost entirely about how good you are with the Rapid Fire Missiles Technique. There are only a few things to know in order to get the fastest fight possible. One is that you do more damage by shooting her in the mouth; there's a faint outline that you can see around the mouth to help with aiming. It's possible to lock on to the mouth by scanning her, but this takes 4 seconds.
You can also control the position of the blue window that protects her by knowing when to skip the cutscene. By letting it play out for a little less than a second, you can make it so that the window is leaving her just as you finish aiming up at her mouth at the end of the L-jump (just as in the above video). However, this method only works if you reach the cutscene quickly enough. If the door closes on you and you end up having to use the morph ball hologram a second time, the whole thing is thrown off. The window starts to move as soon as her room loads, so it's important that the room before her is cleared fast.
Besides that, it just comes down to your ability with rapid missiles. The above video is the fastest one so far (3.8 seconds).

Additional info:

On hard, it's a little different. She has too much health to be realistically killed with just missiles, even if you scan her. If all 15 missiles hit, you can still finish her off with charged shots before the window starts to spin. When you skip the cutscene also doesn't matter in this case, since there is plenty of time to move around while you scan her (see the below video).