The KDX might be decent, but it'll never be fast - it doesn't have CPU power nor enough RAM for proper caching.
You might want to look at kindlepdfviewer (KPV),
https://www.mobileread.com/forums/sho...d.php?t=157047
Note that its development has essentially stopped, all development effort is now focused on Koreader,
https://www.mobileread.com/forums/sho...d.php?t=209276
Koreader has few developers that actually own KDX or K2/K3 devices, but we have reports that the newest codebase should work on KDX. That would be in the next nightly build then, I think.
There's also Kai771's great "Librerator" software, a fork of KPV:
https://www.mobileread.com/forums/sho...d.php?t=198742