[BUG] npm exec --workspaces very slow

Is there an existing issue for this?

• I have searched the existing issues

This issue exists in the latest npm version

• I am using the latest npm

Current Behavior

We have a monorepo using npm workspaces, containing ~70 workspaces/packages.

We noticed that running npm exec --workspaces (for example, npm exec --workspaces --call 'pwd') at the top-level of this monorepo was incredibly slow - the npm process would be pegged at 100% CPU and it would take multiple seconds to print the directory path for each workspace.

The overall run time of npm exec --workspaces --call 'pwd' can take several minutes.

I was able to bisect NPM release versions to identify that this command was fast in version 8.7.0, but slow in version 8.8.0, so it appears that https://github.com/npm/cli/pull/4643 introduced the slowness (cc @ruyadorno). Indeed if I locally revert the change from that PR (modified revert due to subsequent changes from #5250), the performance issue disappears:

--- /Users/gavin/.nvm/versions/node/v16.17.0/lib/node_modules/npm/lib/commands/exec.js	2022-09-14 15:12:56.000000000 -0400
+++ /Users/gavin/.nvm/versions/node/v16.17.0/lib/node_modules/npm/lib/commands/exec-2.js	2022-09-14 15:12:37.000000000 -0400
@@ -25,7 +25,7 @@
 
   async exec (_args, { locationMsg, runPath } = {}) {
     // This is where libnpmexec will look for locally installed packages
-    const localPrefix = this.npm.localPrefix
+    const localPrefix = runPath ? runPath : this.npm.localPrefix

Expected Behavior

npm exec --workspaces --call 'pwd' should very quickly (<2s) print out all workspace directories.

Steps To Reproduce

1. In a large monorepo with ~70 workspaces
2. With a default workspaces config
3. Run npm exec --workspaces --call 'pwd'
4. See slowness

Environment

• npm: 8.19.2
• Node.js: 16.17.0
• OS Name: macOS 12.5.1
• System Model Name: Macbook Pro (2021 M1 Max)
• npm config:

; node bin location = /Users/gavin/.nvm/versions/node/v16.17.0/bin/node
; node version = v16.17.0
; npm local prefix = /Users/gavin/dev/league-web
; npm version = 8.19.2
; cwd = /Users/gavin/dev/league-web
; HOME = /Users/gavin

Start with lib/commands/exec.js and the change introduced by pull request #4643, then reproduce npm exec --workspaces --call 'pwd' in a monorepo with about 70 workspaces. Compare the behavior with npm 8.7.0 and the reported revert; done means all workspace directories print in under two seconds without pegging the CPU.