Best to break apart that compound clip and then select everything in the project and paste it into a new new project. It has been discussion in the past where project garbage builds up in the project file over time especially if you are deleting lots of clips. The project gets a refresh if you copy all the clips into a new project timeline. I tried that out quite a while ago and it did speed up those particular projects and cut down the project file size quite a bit something that was 120mb down to 70mb. Depends on how many changes you have made to the timeline of course.
did exactly that ... splitup the compound clip back to it's original files ... copied & pasted the whole timeline-content into the timeline of a blank new project ... opening that one still takes about 13h (and counting) ... the project file is also only a little bit smaller ... 482mb
On the screenshot you have made I see that you are keeping your Event and your Project on your boot drive (Macintosh HD). That's definitely a big NO for such a long project. In fact you never, ever should put any FCPX Events or Projects on your system drive as this will affect your editing performance.
If you have a fast external drive (a decent FireWire 800 will do for not too complex projects) try this: copy your Final Cut Events and your Final Cut Projects folders from your system drive onto to the external drive. When this is done put your original Events and Projects folders inside a new folder. Call this folder "Safety Backup". Putting the original Event and Project folders inside another one will prevent FCP10 from opening them when you launch the application.
Then, with the external drive attached to your system, launch FCP10. You will now only see the Event and Project folders that are on your external drive. The project should open faster now. If it does delete all your render files to trim the size of your Project, do the remaining work and export without rendering.
13hr+ loading time is still pretty extra-ordinary even if the project and events are on your boot drive(i wasn't able to download the screen shot). a 488mb project file is extra-ordinary for a 90 minute timeline too. I've had a look at all of my own long form projects and all of them are around 100mb for 90 minutes.
VDJ: do you have a huge amount of effects and cuts? I would suggest getting apple to look at your project file if your willing, something is really up with the 488mb project file. You could try breaking up the project up into 10 minute project sections and see if there are some corrupted clips in there or maybe a clip that has a thousand effects on it.
the problem seems not to be the large compound clip itself (as a already split it up), but the fact that there were also a few (very small) compound clips within the project that actually had been put together by compounded clips in the first place ... as i split up ALL the compound clips of the project so far back that they ALL were original files, the project file went from around 450mb to 350 mb and the loading time for the whole monster is now down to a few minutes (which is cool for me to finish the project).
For what it's worth: Compound clips are still a disaster even today in 10.5.1
Even small uncomplicated compound clips can cause FCPX to freeze and even make it impossible to restart it. In my case in some cases when this happens, I must restart my machine. Can not do a restart command but must unplug the machine because t won't restart. My advice: avoid compound clips as much as you can
I use compound clips all the time without problems. It sounds as if there may be a corrupted clip, or clip reference inside the compound clip. Make a new library, import a couple of known good clips and put them on a timeline and make them into a compound clip and see if you still have the problem.