I have never seen a car do this, and cannot find ANYWHERE with the same sort of issue that I have:

Now I will preface this by saying, I know it could be a million things, I am just wondering if anyone else has had this exact issue before.

What happened: driving along at 70 mph, and this jerk is going 55 in a 70 in the farthest left lane, I zip it into 3rd to pass him (I know this is hard on the car, but it was a smooth shift) this is going uphill. I notice my car is making a SUPER LOUD noise, like a sort of sputtering or backfire through a muffler type of sound, I can hear it behind me in the cabin, almost from the muffler. And then I notice the car will not accelerate up the hill. So after cresting the hill, I pass him in 4th gear and zip on my way to work. This has never happened before on the car.
I then noticed on the way home that the car is absolutely fine under 50 mph and accelerates fine in 1st and 2nd, however I can hear what sounds like a hissing vacuum leak that increases with engine RPM.
I had cruise control set to 75 and when going up a (not so sloped) hill, the car would barely stay at 70, and the CC was flooring the throttle.
When im going anywhere above 50 mph in 3rd or 4th gear and i floor it, the car will not accelerate, and will make a horrid noise (if needed i can record this noise for you)

What you might need to know before:
The car will top out in RPM at 4.5k RPM. I have never seen it go higher, I have never needed it to go higher. Im assuming the tach is accurate.
The car has never had any performance, leaking, or any major issues before.
The car has two broken motor mounts still. (both transaxial AKA front and rear of the engine bay)
The car did have a very small exhaust leak before, I could hear it at highways speeds when accelerating hard, I thought this was caused from the motor mounts allowing play in the exhaust.
I can hear what sounds like a belt bearing going bad on the car, not on the AC belt, sounds like either the power steering or the alternator, will need to get a mechanics stethoscope to find out which.
My MPG was around 26, but the most recent one was 21...
No check engine light, and no oil light despite having smelled oil burning when i parked (no leaks either)

What I am thinking so far:
timing belt skipped?

 

Verify the CEL functions, it should come on when turning the key to II(ON).
Verify there are no stored codes, just because the CEL isn't lit doesn't mean there isnt a stored code.
Noise may be a failing bearing in a component, this will cause massive drag on the engine reducing power and economy. Pull off the other belt and see if the engine revs freely.

I have the incorrect transmission in my car and I still get 30MPG, if it was the correct one it would be 35MPG if not better.
26MPG is bad, 21 is horrid.

Check the basics, fuel, spark, air.
When/what was the last maintenance done on the car?

 

As long as I have had the car, never taken it to the mechanics, the PO was a workaround kind of fellow, not a fix it for good type. So im guessing... not in a long long time.
As to your MPG, I got 36 mpg when highway driving, i just drive my car hard, and drive through mostly city.
CEL does indeed come on during start up, only lights that dont come on are the seat belt and fuel warning, will check for codes.

 

No codes, but this morning absolutely no issues... Not even at operational temp... Maybe its one of those one time wonder deals.

 

Check the air filter housing, verify nothing was sucked into the air box impeding air flow.
Does the engine rev freely, or is it still rpm limited?
If your engine is the B1 VTEC, verify the oil is full, low oil/oil pressure will prevent the VTEC solenoid from activating and will limit engine rpm.

 

Mysteriously the RPM is now not limited, previous note on that, the car when i got it idled at really low rpm, i found out that the PO installed the airbox wrong, leaving a huge hole at the front, the air filter was really clean, and the inside of the throttle body was coated in dirt. I got in there and carb cleaned the iac, and everything i could. The airbox is sealed with a fresh filter, ill double check.
Its a non vtec engine.
The car does still have a rough idle, and the idle noise is still there, its like a little chirp, dependant on engine RPM.