When you encounter a bug or something seems to be malfunctioning, your first place to check for more information is the Doomsday log file called doomsday.out. It may contain errors or diagnostic messages that help understand what is going on.
doomsday.out can be found in the runtime folder.
When reporting the bug or posting on the tech support forum, please attach the contents of the log file. This will ensure the basic information like operating system, OpenGL version and driver identifier, Doomsday version and build type, game configuration and loaded mods are included. These details may be crucial when trying to figure out what is causing the issue.
Since version 1.10, the renderer has been inflicted with some performance bottlenecks.
Version 2.1 introduced OpenGL draw call batching, which alleviated some of the issues.
Here are some things you can do to see what is triggering the slowdown: