Yes, light baking is definitely multithreaded. In the old days, you used to be able to see Swarm’s performance per-thread. I take no chances and use an i7, 3.4Ghz, 4 cores 8 threads, boost to ~4.2 Ghz. I was very lucky to get a very good price on a Lenovo Ideacentre desktop.
If you can afford to fork over the $300 for a good i7, you will get the quad core and boost of the i5 plus the SMT of the i3 and a large cache all put together into one amazing powerhouse CPU for a total of 8 operating threads, instead of just the usual 4. i5 is good enough for gaming performance, there is very little to be gained using an i7 for that, but building lighting on i5s can still take a very long time due to the lack of threads.
Remember, a more powerful CPU may even require not just a new motherboard, but a new power supply as well.