The "file name too long" was an issue in npm v2, where each module installed its dependencies inside itself, recursively, leading to enormous node_modules/foo/node_modules/bar/node_modules/baz/node_modules/quux/node_modules/... chains.

npm v3 was an attempt to fix this so it now tries to install all modules and their dependencies in the top level directory, unless there's a version conflict, in which case it falls back to the old behaviour. npm v3 also got slower, because to fix the folder chains it changed how the packages are downloaded and installed.

It sounds like your node_modules were installed by npm v2 – you can try nuking your entire node_modules folder, ensuring you have npm v3, and letting it install your dependencies again. If your guess about grunt's performance problems is correct, this might improve your build experience.