I almost never preinflate. The only time I really consider it is if I'm launching way overpowered or if the launch site is really sketchy and there's no room for error and even then I maybe take 30 seconds to do it. If you are slow and have the wing tip weighed down with enough sand, you can slowly inflate it during launch. If you're kite leaves the ground before you are ready, then you need to weight it down more next time.
The secret to a good quick, controlled launch is having the trim set for full power, and launch the kite with the bar sheeted fully in. This will inflate the foil the fastest and still give you good control. Of course on higher wind launches, be ready to pull the depower once it's pretty well inflated, takes a couple practices to get it right. I do this even with my Soul 15m on 20+ kt days. The Soul does take a little longer to inflate the wing tips compared to say a Sonic 2 just because there's more space.
I use a foil board so not launching big kites in strong wind. I never bother to pre inflate anymore, I lay the kite directly down wind and get all the lines untangled, slowly launch the kite and use plenty of bar pressure to inflate the tips. The Soul is super easy to inflate so you can do whatever you want. With the Sonic 2 or Sonic Race, I would only launch directly down wind. If you launch off to a side, the tip can lift up and go over too far and you have a mess. I get the side launch idea, reasonable idea but is not practical. When you are laying out the kite and making sure the lines are not tangled, by pulling on the front lines the front of the kite will take shape due to the fishing line that is in the kite. If you have an older kite without this, a little pre inflation helps.
Are people launching the kite with the Bar pulled in or out? Which is safer? New to foils and have found my 15m can have a lot of pull on the launch. I have found it seemed to launch better straight down Wind with less pull but have only launched it four times altogether though. Bit worried cause of the power. Think I should of bought a smaller one to start with. May sell my 15m as don’t have much room to launch at my local spot.
Also how you trim it really helps in launching. I always launch with the depower strap trimmed for full power with the bar fully sheeted in. This inflate the kite quicker and allows for more stability and control of the kite during inflation. As the power/pull increases once the kite is close to fully inflated, you can then depower and sheet out as necessary. As above mentioned, go ahead and practice in a big flat area a couple times in varied wind so you know what to expect. I agree that the kite has power much greater than you would expect out of it's size. While I love my Soul 15m especially when over powered, I could have easily been happy with a 12m instead.
You can kinda see how I do this in my video of how long it takes for the 15 Soul to inflate. I sped up the video to help shorter the vid but as you can see the depower is set for full power and my bar is in almost the entire time. Due to wind shadows at that launch there's not much wind at ground level but this is how I do it usually all the time. If the wind is really howling, I'll do more of a 45 degree launch and take my time getting the kite inflated before it completely leaves the ground.