Sure, if the companies you've worked for are in a field that needs incremental performance gains and are willing to pay for it then that's totally rational.
Typically I see client-side performance concerns outweighing server-side performance in a ratio of 70/30 or so, with the remaining server-side performance biased towards I/O concerns like waiting for data, or file system reads with a ratio of 90/10 or more. That puts the actual saving available to language or algorithm changes in the app layer to be less than 3% for the kinds of apps I've worked on.
I usually work at companies who are starting out, looking for Product Market Fit, where those marginal gains aren't worth the cost of reimplementing.
Companies I worked for always had two environments, one for new features and one for performance.
Like PHP and C, new features where implemented in PHP and if they caught on, they got reimplemented in C if they needed better performance.