My question is: Did you use

My question is: Did you use RDTSC instruction to measure performance of these two test-cases?

Here are pseudo-codes:

...
RaisePriorityToREALTIME
EnterCriticalSection
UseRDTSCtoTimeStart
DoCalculationsVersion1
UseRDTSCtoTimeEnd
LeaveCriticalSection
LowerPriorityToNORMAL
Print( TimeEnd - TimeStart )InClockCycles
...

and

...
RaisePriorityToREALTIME
EnterCriticalSection
UseRDTSCtoTimeStart
DoCalculationsVersion2
UseRDTSCtoTimeEnd
LeaveCriticalSection
LowerPriorityToNORMAL
Print( TimeEnd - TimeStart )InClockCycles
...

A verification / comparison of generated assembler codes for both versions would answer your question.