SONAR is taking a long time to launch on my system these days. This morning it was 3:20 from clicking a .cwt file until ready to play. Only two changes to the computer that I'm aware of: the 1709 Windows update, and the installation of three different DAWs (Samplitude, Studio One and Mixcraft demo).
 
Anyone else have this happen, or know of a possible cause?

No where near that extreme but I do believe the MAGIX Low Latency Driver (aka ASIO4ALL) slowed SONAR startup. I removed the driver from the registry. See this post and the one following it.

It's always been slow here (although I never actually timed it), and it can take absolute -ages- to unload too.

I switched to S1 here. I always thought Sonar took long too. S1 takes way longer even for fairly modest projects. Anything with Kontakt takes way longer in S1 than in Platinum. As close to identical projects as I could muster. I'm not bashing S1 as it turns out I like it a lot. But over the years I've heard a lot of comments about Sonar that in my brief time window shopping alternatives leads me to conclude that we Sonar users had/(have) it pretty good. I'm still in the process of moving on but it's not as enjoyable as just using Sonar has been.
My 2 cents

In my experience since jumping ship 2 1/2 years or so ago on the release of Studio One 3, my experience has been the opposite. Regarding startup, load speed, and speed in general, Studio One 3 makes SONAR look like it's crippled.

I see there is definitely a trend and conclusion developing here .... NOT

Sonar opens for me in 10 seconds. Every time.  I disable the Plug in scan. Why scan plug ins if you haven't installed any? 

Just some thoughts here:
 
Does this only happen with that template, or does it take ages with a blank template too?
 
Have you blocked internet access, and is there any plugin (or even more than one plugin) in your template that is trying to check for authorisation/updates? It might be that the cumulative time-outs of each plugin giving up is causing the slow startup?

One more thing - if this has happened since you installed Samplitude, Studio One and Mixcraft, you might want to go to SONAR's plugin manager and exclude any new plugins these DAWs have introduced. Maybe one of them is causing the issue.
 
I've got to say though, I've recently installed the same three DAWS (Samplitude X3 Pro, Studio One 3 Pro & MixCraft 8 Pro) and I'm not seeing any slow down in SONAR's startup time. In fact, SONAR is the quickest out of all of them to start up.
 

UPDATE:
I disabled a bunch of nVidia HD audio drivers -- don't remember how many, at least eight -- and SONAR is starting much faster now. I now believe the Windows 1709 update caused a large part of this. Maybe I'll get around to removing the Magix Low Latency driver but so far I have not found an uninstaller for it.
SONAR may not be a fast starter, but I'm a patient guy, as long as it's responsive once it's open and I'm using it. And it is.
But I am currently devoting all my DAW time to learning Samplitude, which I have found to be kind of a bear. I want to be as facile with it as I was getting to be in SONAR, and it's just going to take time. I have a few old projects in SONAR that probably will need remixing some day, so I'm keeping it. I love it, but it's an Edsel.
Thanks scook for your advice.