You can check performance by yourself.
clangd:

--compile-commands-dir=<string> - Specify a path to look for compile_commands.json. If path is invalid, clangd will look in the current directory and parent paths of each source file

ccls:

--index=<root>        - standalone mode: index a project and exit

Do not forget to run "umk -j" to create a compile_commands.json file.