ts-ls slow to insert completion because of "completionItem/resolve"

[rafael-unloan]

Thank you for the bug report

• I am using the latest version of lsp-mode related packages.
• I checked FAQ and Troubleshooting sections
• You may also try reproduce the issue using clean environment using the following command: M-x lsp-start-plain

Bug description

I noticed that particular completion items take 1.5 - 2 secs to be inserted.
I traced it down to this:

               ;; see #3498 typescript-language-server does not provide the
               ;; proper insertText without resolving.
               (if (lsp-completion--find-workspace 'ts-ls)
                   (lsp-completion--resolve candidate)
                 candidate))

Pretty much 99% of the 1.5 - 2 sec latency is from lsp-completion--resolve
I know that this was added to support text edits (e.g. auto-imports) etc, but I wonder, how does VSCode do it and not lag for 1 - 2 secs?

Screen.Recording.2025-04-11.at.10.31.30.PM.mov

Steps to reproduce

Try inserting completion items for a Typescript project

Expected behavior

Completion should be almost instant

Which Language Server did you use?

ts-ls

OS

MacOS

Error callstack

N/A (performance issue)

Anything else?

No response

[kiennq]

Probably we can make the resolve item async on completion completed as well. WDYT @wyuenho?

[wyuenho]

@rafael-unloan if you trace the issue number back, you'll know this is a particular quirk of ts-ls. The reason VS Code is faster at completion is because it doesn't use a language server for Typescript, it just uses tsserver directly, which is not the same thing and does not require resolution for any insertText. In an effort to emulate tsserver's bahavior under LSP, typescript-language-server opted to require resolution for the actual text to be inserted. This isn't something lsp-mode can and should optimize for, as resolving async eagerly will overwhelm ts-ls even more, and every textDocument/completion request will take 200+ms to respond if you type just a little bit faster.

The fastest Typescript server Emacs package is tide as it sidesteps all this LSP overhead and language server feature mismatches. Give it a try.

[rafael-unloan]

I see, thank you for the info @wyuenho! I've tried before but still prefer lsp-mode.

If I disable (lsp-completion--resolve candidate) (via an advice in my personal config), does it just mean that I don't get auto-imports? Or are there other downsides?

[wyuenho]

You probably will lose a good chuck of completion candidates

https://github.com/typescript-language-server/typescript-language-server/blob/master/src/completion.ts#L133-L136

[wyuenho]

@rafael-unloan see if this snippet will make lsp completion faster, this is how you lose auto-import but speeds up everything else that's not a function, method or a snippet. I still don't know how much this is actually going to speed things up overall tho.

               ;; see #3498 typescript-language-server does not provide the
               ;; proper insertText without resolving.
               (if (and (lsp-completion--find-workspace 'ts-ls)
                            lsp-enable-snippet
                            (eql (lsp:completion-item-insert-text-format? item)
                                 lsp/insert-text-format-snippet)
                   (lsp-completion--resolve candidate)
                 candidate))

[wyuenho]

Apologies for the earlier mix-up. I've made a PR with that snippet above, that should skip sync resolve unless for function or method completions. Auto import won't go away, the additional text edits and commands are resolved async later in the exit function.

[rafael-unloan]

Thank you @wyuenho! I'll try out #4763