1. It's a hobby in your own right. The nasty is not that bad, back in the day the annoying thing was how fiddly it was but the community thought of better ways like etching with a sponge. You can get better boards made online but that applies to all hobbies.
2. In most of US/EU you get them for $5 in up to _two weeks_, or in two days for $5+$2X or more which adds up quicklyand can be more than the rest of the project. Even next day shipping loses out badly to making it yourself in ~30 minutes, turns into two days delay unless you're lucky to have an idea in the morning and time to work on it in the next afternoon, a lot of the time i see people bodging things i'd make another board for because it's just easier.
And multiplication is a fancy convolution (with carries).
Fast convolution algorithms use FFT and run in O(n log n) time, which is why fast multiplication algorithms are based on FFT and why everyone was looking for O(n log n) one, which was found like a year ago.
(Though from what i can tell in "Faster Convolutions" section the article claims you can easily build O(n log n) multiplication using FFT which doesn't work as it ignores carries)