You can add tons of explanatory comments with zero performance cost.

Also in programming in general (so, outside stuff like being a Quant) the fraction of the code made which has high performance as the top priority is miniscule (and I say this having actually designed high-performance software systems for a living) - as explained earlier by @ForegoneConclusion, you don't optimize upfront, you optimized when you figure out it's actually needed.

Thinking about it, if you're designing your own small matrix multiplication library (i.e. reinventing the wheel) you're probably failing at a software design level: as long as the licensing is compatible, it's usually better to get something that already exists, is performance oriented and has been in use for decades than making your own (almost certainly inferior and with fresh new bugs) thing.

PS: Not a personal critical - I too still have to remind myself at times to not just reinvent that which is already there. It's only natural for programmers to trust their own skills above whatever random people did some library and to want to program rather than spend time evaluating what's out there.