Does restarting SQL Server speed it up? I have noticed that some DBAs restart SQL Server very frequently, sometimes even nightly. I believe they do it to free up some memory, or perhaps to speed up queries too. I know that after a restart query plans have to be recompiled, but even including that I wonder if there is a net benefit to this practice.

Is it true that restarting SQL Server daily makes it run faster?
Restarting the server is probably one of the most damaging things for performance. It means you force a cold cache for data, a cold cache for query plans, and all SQL Server internal caches are also nuked in the process. Not to mention that by throwing away all the statistics collected in the operational stats DMVs, you diminish your chances of ever investigating something successfully.

There is no official guidance backing this practice, I have never seen it mentioned in any good practice reputable work, I never hear of a reputable expert ever mentioning this as a practice. Don't do it.