Spread SQL Server data files across devices to improve performance? Would it improve performance of a database in SQL Server if I place the .mdf (rows data) and .ldf (log) files on separate sets of raided disks, as opposed to having both files on the same set of raided disks?
All performance solutions are based on measurements. As the theory goes, SQL Server requires the transactions to flush to durable storage (disk) before acknowledging commit (WAL). This makes the log write throughput critical for high write loads. Separating the LDF file on a separate physical disk allows the log writes to achieve serial streaming access to disk, which is the fastest way a spinning disk can write (~100-150Mb/s). Having the MDFs on the same physical disk introduces random IO patterns that dramatically reduce the throughput (~10-40Mb/s). This is the background of the separate LDF /MDF disk recommendations. The point is, obviously, mute on SSD storage.

However, to improve the server performance you need to identify the bottleneck and address it appropriately. If your bottleneck is poor indexing, lock contention, CPU, small buffer pool or any other of many more possible causes then separating the LDF and MDF will help nothing. So you should first find the bottlenecks, and I recommend following the Waits and Queues methodology. Once you have measurements that prove a bottleneck that causes performance degradation then and only then you can act appropriately.