Ive been using Unity for about 2 years, every month or so I get an hour or 2 where Unity will run super smooth and it’s a complete joy to use, even on my HUGE scene, which is over 30 megs just for the scene file - it’s as fast as it would be in a fresh almost empty scene. I can zoom right out to show thousands of buildings and sprites etc, no slow down at all - usually this would be slow as can be, and I would need to go through turning areas of my level off to get a responsiveness. But just as quickly as it arrives, it will go away, leaving Unity feeling very sluggish again. When it happens it is like magic 
I’m wondering what the cause of this could be and how I remain in super fast mode all the time
Sounds like your computer running out of memory.
No it isnt that, sorry I should have mentioned that I have 16 gigs of ram, I usually have lots of ram left even when its running slow, like now Im at 73%. It’s gone back to being slow right now, it was fast earlier, I wont see super fast mode for a month or so now. I will update all ym graphics card stuff it must be that thanks.
I always think of the simple things first.
I agree you should have plenty of ram to handle Unity and a few other things all at once.
Good luck with figuring this out.
Make a bug report with your 30megs scene (or your entire project if isn’t too big). That would be a very good test case for Unity.
Unity is a 32-bit application, so will only use 2/3GB of your 16 gigs.
It won’t be able to use most of that until Unity 5 when it’ll gain 64-bit support.
It’s possible that you have something in your assets folder that’s dragging it down. I use 4.x but I have experienced that happening with 3.x in the past. Some script or dll or shader or something that’s not even being used in the game and somehow Unity still keeps seeing it and choking. Do a cleanup of your assets folders and get rid of things you’re definitely not using. Hopefully that will help.
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What’s it do if it needs more than that? Crash, or swap something to disc?
Crash. 32-bit software typically hard limit on memory. It makes no difference weather this is real memory in ram, or fake memory on the hardrive. There are only so many addresses available.
If you have Unity Pro, check the profiler.
I have a crappy lap top (8 gigs of ram) an I’Core 3 2.4 Ghz Dual Core.
I never expierience any of these fast and slow streaks.
I have however, had something in Update() that I forgot to remove like instantiate a projectile that never stopped and bogged Unity out, aside from that never had an issue before.
Tried my project on Unity5, the profiler might have something more to it, because I immediately found 2 main problems why this was slow, it is now ALWAYS in superfast mode, ie, normal mode everyone else has been enjoying!
It seems USequencer (an asset for cutscenes) was draining my cpu with many calls to something called “USTimelinePropertyEditor” - and further still my own script I had written to tweak the UI somewhere for drawing custom gizmos!! Its easy to cripple Unity and not realise, I use the profiler every day so Im confused why I never found this before, as I say Unity5s seems better!..
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That limit isn’t so hard if you can use a paging system, it just typically makes things too unbearably slow for use in a real-time application like Unity. It’s cool for something like Photoshop to page in and out as you swap layers or do individual operations, but it’s really not ok for Unity since it’d potentially have to do it every frame.
30 megs?
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Could it be, its that time has slowed down only for you, so everything else moves faster, relatively? XD
I have this same issue, but generally with script recompilations. if I delete my library folder and start unity, all my scripts recompile really really fast. After a few hours, even if I make the smallest change to a script, it starts taking like 30 seconds to recompile. There’s a bug somewhere, its a bit irritating as I tend to change something, test the change, change it again, test it again, at a fairly fast pace. Its pretty irritating to have to stare that the spinning circle for 30 seconds while I wait for my Debug.Log to be ready.