We switched our monorepo from yarn classic to pnpm and the install speeds have been night and day. Sometimes it feels like I ran the wrong command because its so fast.
pnpm also helped us solve issues with packages in our monorepo importing dependencies declared in other packages because of node_modules hoisting behaviour with yarn. With yarn, all of your packages' direct and indirect dependencies can be resolved from any package in your monorepo. This makes it hard to isolate packages from each other. For example, this was causing issues when we wanted to generate docker images for some of our packages since we only wanted to copy specific directories into the image rather than the entire monorepo to avoid having to bring the entire monorepo to the image. pnpm also allows us to use features like git sparse-checkout because we can be confident there are no implicit dependencies between packages.
pnpm makes this possible by only exposing in the node_modules direct dependencies. In monorepos that don't use pnpm, you can remove a dependency and your monorepo can break in unexpected places because other packages in the repo implicitly depend on this dependency. This is more common than you think because people tend to rely on IDE auto-completion when writing imports and IDEs just tend to read node_modules instead of package.json so its very easy to take on an implicit dependency when all directs/indirects are hoisted into a root node_modules.
Yarn supposedly fixes this issue in their newer releases with Plug n Play but from from what I understand it basically monkey patches Node's require statements with its own behaviour. This just didn't feel good to me from a tooling compatibility perspective which is why I went with pnpm and I'm glad I did. I'm honestly puzzled at Yarn's popularity and why pnpm isn't as popular as it should be.