Rarely executed and almost empty if statement drastically reduces performance in C++

I have a simulation where if i add a simple if statement into the while loop that runs the actual simulation, the performance drops about an order of magnitude (and i run a lot of calculations in the while loop, n-body gravity for the solar system besides other things) even though the if statement is almost never executed:

if (time - cb_last_orbital_update > 5000000)
{
    cb_last_orbital_update = time;
}

with time and cb_last_orbital_update being both of type double and defined in the beginning of the main function, where this if statement is too. Usually there are computations i want to run there too, but it makes no difference if i delete them. The if statement as it is above has the same effect on the performance.

The variable "time" is the simulation time, it increases in 0.001 steps in the beginning so it takes a really long time until the if statement is executed for the first time (I also included printing a message to see if it is being executed, but it is not, or at least only when it's supposed to). Regardless, the performance drops almost an order of magnitude even in the first minutes of the simulation when it hasn't been executed once yet. If i comment out the line

cb_last_orbital_update = time;

then it runs faster again, so it's not the check for

time - cb_last_orbital_update > 5000000

either, it's definitely the simple act of writing current simulation time into this variable. It get's better though: Sometimes when i change things like the > to >=

if(time - cb_last_orbital_update > 5000000)

to

if(time - cb_last_orbital_update >= 5000000)

it runs faster again (this doesn't always work though, but i can't see a pattern). Also worked with changing 5000000 to 5E6 once. It only runs fast once though, recompiling causes the performance to drop again without changing anything. One time it ran slower only after recompiling twice.

Also, if i write the current time into another variable instead of cb_last_orbital_update, the performance does not drop. So this might be an issue with assigning a new value to a variable that is used to check if the "if" should be executed? These are all shots in the dark though.

Disclaimer: I am pretty new to programming, and sorry for all that text.

I am using Visual C++ 2010 Express, deactivating the stdafx.h precompiled header function didn't make a difference either.