On a daily basis I use a Raspberry Pi v2 (4x900MHz) with Raspian as a work station and web server. It is connected to a big display, I edit multiple files and it runs multiple Node.js instances. These Node.js processes serve HTTP and access (both read and write) local files.
I experienced regular freezes. Things that could take 2-3 seconds were listing files in a directory, opening a file, saving a file and so on.
I moved my working directory from my (high performance) SD-card to a regular spinning USB hard drive. That completely solved the problem. I experience zero freezes now, compared to plenty before.
My usual experience with Linux is that the block caching layer is highly effective: things get synced to disk when there is time to do so. I dont know if Linux handles SD-cards fundamentally different from other hard drives (syncing more often) or if the SD card (or the Raspberry Pi SD card hardware) is just slower.
So, for making real use of a Raspberry Pi I would clearly recommend a harddrive.