You have an i7 with twice as many cores so it should be way faster than that - you might be able to encode an hour of video in ten minutes!
The i7 has exactly the same number of cores as the i5, except the i7 has 2MB extra L3 cache and has HyperThreading enabled, which lets each core run two threads for better thread-level parallelism and more efficient use of execution units within each core.
That's what I meant to say - it can do nearly twice as much work as an i5.
I think each pair of cores has two execution units but a single floating point unit. Heavy floating point math is a teeny bit less efficient with hyperthreading than with separate cores.
The FPU works like a FIFO: You put numbers in the front and 'X' clock cycles later the result comes out the other end, with 'X' depending on the operation.
If two threads are putting numbers into the FIFO then single cycle instructions like addition can take a hit because they have to alternate. OTOH instructions which take many clock cycles (multiply, divide, sqrt, sin/cos, etc.) will make much better use of the FPU because with two threads you can have twice as many of them passing through the FIFO at the same time.
In reality the effect on single-cycle instructions isn't too bad because it's quite difficult to put a new number into the FIFO on every single clock cycle - you need to store results, fetch new operands from RAM, etc., this frees up the FPU for the other thread. With the rigth code they can interleave perfectly with no clashes.
(Or at least, that's how "hyperthreading" worked when I was younger...)