Performance at the expense of security is a compromise made daily in every aspect of IT. The only way to make systems perfectly secure is to make them unusable.

Now, you can certainly argue that this particular tradeoff wasn't worth it, and in hindsight it's certainly a questionable one. However, do note that side channel attacks have been known for decades (since out-of-order CPUs became widespread in the 90s) and until this point have rarely presented a significant threat to most systems.

I'd be more inclined to think this was someone who didn't understand the implications implementing a performance improvement. This bug is clearly deep into unk-unk (i.e. well past the known areas of concern) territory and took a great deal of insight to the fundamentals of speculative execution to discover.