Your diagnosis of highlight group clashes is precise, but the underlying cause is often more structural. The `hi link` cascade usually happens when plugins load asynchronously or in an order-dependent way, especially with lazy-loading plugin managers. It's not just two plugins defining the same group, but one plugin redefining the syntax region itself after colors have been linked.
I've found that the freeze from multiple LSP clients is indeed an event loop contention, but profiling with `--startuptime` often misses it because the conflict happens during runtime event storms, not startup. A more reliable method is to run Vim with `vim -V20vimlog.txt` and then trigger a completion, then grep the log for `UIEnter` or `autocommand` loops. You'll frequently see the same event firing hundreds of times as two clients fight to provide completions.
Your point about Emacs having uniformly bad slowdowns versus Vim's intermittent freezes is a key operational distinction. That predictability, while painful, is often easier to diagnose and budget for in terms of system resources. The Vim case becomes a latent fault that only triggers under specific multi-filetype workflows, making it a nightmare for shared team configs where usage patterns diverge.