If you're consistently taxing the four cores on the i7, you can turn on HT and it might actually help. This isn't your father's HT, it's a completely new implementation and really only shares the name with the predecessor. With the current HT (SMT), you can run two threads at full speed on one core. The threads actually run at the same time. What you lose now with HT turned on is the additional ALU for processing the instruction stream. With HT off, the one thread on each core gets two ALUs, just like it's always been. And almost all of the time, one or both of those ALUs are stalled. With HT on, each thread on each core gets a single ALU, but the rest of the control pieces are duplicated. You'll stall the ALU less, but instructions streams that could have been parallelized between two ALUs will need more clock cycles to complete.