Orion doesn't take long, it opens pretty much instantly.
Not if it hasnt scanned plugins yet. (I.e. the first scan after OS boot/reboot always takes long - how long exactly depending on the amount of plugins installed.)
Once it did that first/initial scan, the operation sorta remains 'cached'. Thus from that point forward Orion will start more or less instantly since it doesnt need to scan everything from scratch. However, should that caching get eliminated by other stuff happening on the machine (like for instance loading a game or some other resource-intensive application) it will take long again since a new from-scratch scan needs to be performed.
(The same by the way applies to all hosts that scan plugins every startup. Hence if you run say REAPER, and it scans the plugins, and you then run Orion, Orion will start fast too since the scanning operation has already been performed. (I.e. it doesnt matter what app is doing that first scan. Once the operation is performed/cached, all hosts that scan on startup will benefit from it.))
I'd be surprised if XT2 was faster.
As has been mentioned, eXT doesnt scan plugins ever, it merely lists them as files.
It therefore always starts instantly, only limited by the speed of your machine.
(On a halfways decent PC it wont even take a full second.)
On Topic: I would say Orion too. Cant beat this for speed and immediacy. The concept of each instrument having its very own multi-pattern sequencer plus a 'playlist' where you then arrange the patterns into songs is just what the doctor ordered if youre into sequence-based music. The only one that comes close conceptwise (at least as far as im aware) is Phrazor because it functions in a very similar way.