(Moving this to modded because of the increased ram allocation as a baseline.)
The screen shake is a completely unrelated issue, and is fixed for the next release.
Are you *sure* you're seeing 30 second save times in vanilla? What's a "certain time" - I could see this happening, say, if the game is left to idle for 12 hours or something; it seems like the OS does some funny things sometimes if that happens. But not under normal circumstances; I'm not aware of anything like this in vanilla.
Re: memory usage spikes - how are you measuring this? Task manager is unreliable and basically useless for measuring Java memory use; it's unfortunately just going to produce near-meaningless results.
That said, saving does increase the memory use by some significant amount; nowhere near 6gb in vanilla, though - for vanilla, the standard vanilla allocation of 1.5gb should be a completely fine. (But, again, you might see 6gb in task manager, and that wouldn't mean the game was actually using all of it. And if you allocate more, it might use more than it would if you didn't; it's just not the most straightforward thing.)
And with all of *that* said, you could give this a shot:
https://fractalsoftworks.com/forum/index.php?topic=29320.0