I can't speak to what's normal (you should do a poll for that), but 15-25 seconds doesn't sound abnormal for a SATA hard drive.
As I understand it zone load times are based entirely on how fast your computer can read the zone files (on your hard drive) and then shove them into the appropriate places in-memory (in your RAM). As a result, you have essentially three options:
1. Keep your current hard drive
2. Replace your hard drive (or add a new one) that's a Solid State Drive (SSD) which can read data much faster than older SATA hard drives
3. Use a tool like RAM Disk to make a fake "hard drive" with memory; this will be the fastest option possible
Here's a random comparison I found to give you a sense of scale of three different options:
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Scanning your hard disk for errors and defragmenting it might also help a little, if you really don't like options #2 or #3, but I wouldn't expect it to help that much.