from what I've read, responsive screen refresh is inversely related to battery life. A more responsive screen results in less battery life. Amazon, and pretty much all other eink readers have prioritized battery life over absolute responsiveness.
They have made significant improvements in responsiveness over the years. Do you remember how slow screen refreshes were on the original Kindle? It's just that that is not a high priority for their main use case of linear reading.
If you want to use it for reading PDF reference books, you probably should look to one of the eink Android tablets that are more general purpose devices and may have a faster refresh rate.
There's also an inverse relationship between response time and image quality with e-ink, speeding up the display comes at the cost of more smearing/ghosting. There's often a software option to tweak that balance one way or the other depending on your preference.
They have made significant improvements in responsiveness over the years. Do you remember how slow screen refreshes were on the original Kindle? It's just that that is not a high priority for their main use case of linear reading.
If you want to use it for reading PDF reference books, you probably should look to one of the eink Android tablets that are more general purpose devices and may have a faster refresh rate.