As Grant mentioned, the memory isn't the issue.

Slowness ~!= memory. They may be related, but may not be. Likely if your server is slow, it's not because SQL is using the memory. It's because SQL needs more memory, or you have poor code/indexing/IO.

What's target memory?

What does slow mean? Do you have specific queries that are slow? Have you looked at plans or examined waits as Grant mentioned?