a timing or voltage dependence on the key used for the encryption/decryption is often exploited by side channel attacks. this dependence can be used in some sophisticated attacks to reduce the key space dramatically. c and asm/microcode are close enough to the hardware that the operations let you analyze potential side channel leakage. for instance, if i look at the generated assembly it's possible to reason if the time/voltage used by the decryption operation is a function of the key used in the decryption.

when a virtual machine is introduced the just in time compilation itself may introduce these dependencies between timing/voltage and the key used in decryption even if the source code does not appear to. however, unlike in c, you can't just go examine the compiled assembly to ensure a timing attack wasn't added by the compiler. the actual native code becomes a function of the virtual machine installed.

the guy above you knows what he's talking about.