I’m not exactly sure what you’re worried about here. Usually if Mail crashes, you can just relaunch it, and everything will be fine. Your messages are stored in individual files and written to in a way that there shouldn’t be any data corruption even if Mail crashes. If Mail’s database gets damaged, it can be rebuilt from the message data. Regardless of whether Mail is acting funny, you should still have a backup.
In any case, I don’t think there’s a connection between launch time and Mail crashing.
There’s no such thing as an OK or not OK size.
As I said in the other thread, I recommend that you measure what’s happening so that we can take a look at what Mail is doing, rather than speculate.
Yes, you could do that. It’s possible that the problem will go away and not come back when you reinstall. It’s also possible that it will go away and then come back, and then you’ll be back at square one where I ask you to record a sample.
You could leave the SpamSieve app installed while uninstalling the plug-in. Then just reinstall the plug-in (and recreate the rule) later.
You can enter “Mail” in the search box in Activity Monitor. Then click the Mail icon in the Dock, click the Mail item at the top of the list in Activity Monitor, and choose Sample Process. That should still leave it at least 10 seconds to record the sample.