Processing time/lag when hitting play seems completely random?

Try the reset button on Audio & MIDI Setup window. This will recalibrate the timing with your DAW.

The reset should also recalibrate that. Give it a try.

The spinning wheel of W7 might indicate that something jams on your machine when playback is started. The long delay until Synfire receives a confirmation back from the Engine or DAW makes it think that host is utterly slow in responding, hence the lag of the graphics display.

Do you have network drives, sleeping drives that need to wake up, or similar things in your system? Anything that could cause your DAW to respond slowly?

Do you use a multi-client ASIO driver? Could this possibly jam the DAW, because it conflicts with other audio software also using the driver at the same time (including Synfire's Engine)?

Since you are seeing the spinning wheel on start, I'm pretty sure it's the DAW refusing to start immediately which causes this issue. We just need to find out why.

How fast does the DAW start when you start it independently of Synfire?

A little preroll is required to synch with the Engine, although it should not be that much visible.

Most importantly the spinning circle is gone now. That will make a difference. Synfire recalibrates the transport from time to time. The faster the Engine responds on start, the tighter it will set the timing between the programs. You can not really influence this tuning, although a Reset in the Audio & MIDI Setup window might help (transport button).

Thanks for the videos.

True, it works more responsive on OS X, but that's because audio is deeply integrated with the OS.  The pre-roll in your video isn't too dramatic, as long as the music is in synch. I'll have a look at the code and see if the frequency of recalibrations could be increased.

This morning all of a sudden the global rack was empty.

Hm. That frightens me. I've not seen this yet. Are you sure? Could it be you filtered the rack modules by selecting an unused engine tab? This will also make it appear empty.

If it comes up empty, it must have been saved empty. Did you accidentally load another rack before quitting? Or hit File >> New? If so, your rack is still on disk. You just need to open it.

Concerning the spinning circle, is there any heavy MIDI CC data in your arrangement? It needs to be pushed to the Engine before playback starts. This can cause a delay.

If so, try muting that instrument and see if the circle disappears. Thinning out MIDI CC might then help.

I´ll just save every global rack in its respective project folder 

That's not a workaround, it's recommended. You can share the rack with multiple projects, but keeping a copy of it with each project ensures you can restore an old project later at any time.

I just quit SF, rebooted the computer and there was not plugin present in the global rack anymore.

Just the plugins not loaded, or all rack modules gone, nada?

Well, this indicates a different issue.

Opening the Console after startup from the Window menu will show you all  recent warnings and error messages. Some messages may point you to the cause. 

I just quit SF, rebooted the computer and there was not plugin present in the global rack anymore.

Did you know there is an item Rebuild Rack on the rack's utility menu? This will also reload plug-ins. If any error messages occur, you will learn why plug-ins don't get loaded.

Update: This issue has been resolved (next update).

Here's a trick that should work as a workaround:

1. Set the device latency of "Internal GM Synth" to zero.
2. Save the device
3. Restart Synfire.

If you find any other devices with latencies > 50 ms, do so for them too. Also check if you have those in your arrangement rack.

Of course, when using these devices, you will experience out of synch sound. I assume however that you will not use the Internal GM Synth anyway, because of its awlful latency, so just don't assign it to any of your global instruments and you are fine.

Scan all devices using the arrow keys on the list. Some will have a latency of at least 100-200ms.

Also look at devices copied into your arrangements.

The next update will be soon. Days, not weeks.