Displaying PDF files with many pages leads to an internal call of convert which can last very long (a few minutes). During this time, my Windows server was fully charged to capacity (100% CPU load) by the parallel process of that call of convert.exe. Convert.exe is also very memory hungry and after a few moments the whole RAM was allocated by convert.exe. This is critical for a responding Trac service, all web interface reloads tend to suffer from timeouts.
You should set a priority below "normal" for the external call of convert.exe, and maybe limit the memory for that process.