Yes, this can definitely be a problem. I worked on a transaction processing system that was entirely based on a in-house memory mapped database. All reads and writes went through mmap. At startup, it read through all X gigabytes of data to "ensure" everything was hot, in memory, and also built the in memory indexes.

This actually worked fine in production, since the systems were properly sized and dedicated to this. On dev systems with low memory and often running into swap, you'd run into cases with crazy delays... sometimes a second or two for something that would normally be a few milliseconds.