@aeva maybe uncalled for because I don't know the exact situation you're in, but I have noticed that the choice of reader (software) can be a night and day difference when it comes to performance.
I have an inkbook ereader (slimmed down android thingy). The pre installed reader app is slo, and even slower on pdfs.
I tried KoReader recently and wow. PDF pages load almost instantly. It is not a limitation of the display, the software was just... Slow