While I agree and personally do get frustrated with poor performance web apps... there are few companies that let you focus on performance over features. Throughout my (admittedly short) career all I've heard is "we can optimize later", "product is more important", etc.
Which I'm prepared to admit may be true. It's just boring as fuck.
Seriously, what you think is causing slowdowns will not be at all. Everything you'll be pushing for will save nanoseconds or 10s of milliseconds, pointless, unnecessary complications.
Generally in a crud app performance problems are caused by a single sql statement or a silly, unnecessary, loop within a loop.
Source: 15 years crud app dev and a lot of fixes for clients of their slow apps.