A common and easy-to-fix cause of performance problems is the length
of the Eglot events buffer because it represent additional work that
Eglot must do. After verifying Eglot is operating correctly but
slowly, try to customize the variable eglot-events-buffer-size
(see Eglot Variables) to 0. This will disable any debug logging
and may speed things up.
In other situations, the cause of poor performance lies in the language server itself. Servers use aggressive caching and other techniques to improve their performance. Often, this can be tweaked by changing the server configuration (see Advanced server configuration).