According to your description, it seems a program with mkl_rt.lib(the import library of mkl_rt.dll) or without it run in different speeds. but you haven't mentioned, what else mkl libraries you are linking. Considering the speed difference, we guess it is multi-threading issue. How many CPU on your Pentium IV machine (or HT is no or not?). Could you please provide us a small test case to show the problem?
Regarding mkl_rt.lib (& mkl_rt.dll), theyare new library we introduced in MKL 10.3 as the article
A New Linking Model-Single Dynamic Library mkl_rt Since Intel MKL 10.3
link mkl_rt.lib(mkl_rt.dll) almost = mkl_intel_c_dll.lib mkl_intel_thread_dll.lib mkl_core_dll.lib libiomp5md.lib. as we know, dll have load overhead than static library,but the overhead should not so big (3 to 4 times). So we need to know the details of MKL static library in your program".For example, Doyou use theparallel or sequential library etc.
Here is MKL link Advice: MKL Link Line Advisor