I have been trying wayland (nvidia drivers from rpmfusion; F39 workstation) with mostly good results. The major exception has been Calibre. Viewing any epub is almost impossible due to very slow reponse. Many “E-book viewer is not responding” pop ups. Is this a known issue?
thanks
No, not that I have seen. 
What version of calibre? rpm or flatpak?
Thanks Kevin.
rpm: calibre-6.28.1-2.fc39.x86_64. Fedora 39 workstation all current with repository. rpmfusion nvidia 545 rpms. If use gnome xorg calibre works well, i.e. fast without triggering the not responding pop ups.
I did some additional research and found a promising post at: MobileRead Forums - View Single Post - Ebook-viewer does not work with Ubuntu 22.04 Wayland [6.13].
In short, launch calibre with this setting:
QT_QPA_PLATFORM=xcb calibre
When I start calibre as above calibre seems to work at normal speed.
thanks