I tried to reproduce the issue by using UES v18.104.22.168 on Windows XP.
I opened a C++ project with exactly 500 project files by pure. 243 project files are *.cpp files, some more are *.h files and a few are *.rc files.
I opened next 43 *.cpp files and although using also the function tips data for the function list, the function list worked fine and always displayed immediately the functions for active file.
But then I clicked with right mouse button into the function list view and enabled option List for all Project Files. This was no good idea. UEStudio started the background thread collecting the data indicated by an sand glass mouse pointer, but UES never finished the task and displayed all functions from all project files. UEStudio was still responsive, I could run a Find in Files listing all project files in output window and make also changes in the opened files. But UES executed everything very slow.
So I started Process Explorer from Sysinternals and looked on the process data of UEStudio. It consumed permanently less than 0.4% CPU power. So UES did not run into an endless processing loop. The working memory consumption first increased in small pieces until 37.4 MB memory was used. Number of handles, threads, user and GDI objects were constant. I exited Process Explorer and started Process Monitor to watch registry and file system activities. But the only activity I could see was the periodic check on modifications on the 43 opened files done by UES using another thread. There were no accesses on symbol database files in cache directory. Well, the symbol database should be already in memory of UEStudio and therefore there is no real need to read the cache. I started writing this post in the meantime and as I looked again on UEStudio process in again started Process Explorer, the working memory valued was decreased in the meantime to 32 MB and started again in increasing in small pieces. I clicked on exit icon of UES as after several minutes I did not want to wait longer for the list, but UES did not terminate. I needed to kill UEStudio.
Conclusion: It looks like UEStudio with using function tips data for function list has a thread management problem when number of project files is quite large and List for all Project Files is enabled for the function list.
There is no problem with usage of List for all Project Files when having Use function tips data (if available) for function list not checked in configuration. In this case UES searches in all project files with the regular expressions in the wordfiles for the function strings which is indicated on left side of the status bar. That took several seconds for my project with exactly 500 project files, but the sorted function string list was displayed finally.