Here is a question to which I strongly suspect I already know the answer.
There is a long-running series of complaints by users of McAfee consumer AV products that mcshield, the on-access scanner and so central to the product, performs extremely badly on many older machines, especially those with Windows XP. Most, but not all, of those older machines seem to have Pentium P4 processors. Ignoring for now the inconvenient minority with multi-core processors and/or hyperthreading, that means that most of those with complaints about mcshield have a PC with a single-core single-thread processor.
The performance issues break down into two groups : memory usage, and extremely high CPU usage for extended periods. We can ignore the memory issues. It's the high CPU usage which is cause for concern. It doesn't show up, as a rule, on newer PCs with more than a single core; I can't say whether anyone with a single-core machine with hyperthreading is affected, but none of those who have mentioned their system specs seem to have hyperthreading so it's possible but unproven.
The program code for this product is written, as far as I can find out, in a variant of C - C, C#, or C++. It is almost certain that the code is written for a multithreading architecture and/or has been compiled with the appropriate compiler switches set to optimise the code for multi-core machines and multi-threading. That's just a guess, but it's probably correct.
So program code written for a multithreaded application, intended for the latest multi-core processor machines, is being run on older machines with "legacy" architecture. It's easy to put two and two together and say that when such a program takes over a PC for five or ten minutes at 90% CPU usage it must be because it was never intended to run on an older architecture, but I don't know enough to say that this is definitely so.
So the question in the Subject Header is quite an important one, and I hope that someone here can give a (fairly) concise explanation. What exactly would happen when you run a multithreaded program on an old single-core, single-thread P4 machine (even one with a 3GHz processor and 3Gb of RAM)?
I look forward to reading the contributions from anyone who can give an explanation.