Not too surprisingly I agree with what the other people have said (I'm the ZAP project lead;).
However there are still various options available to you.
One option is to disable any of the ZAP scanning rules that do not apply to your site, or that you are not worried about. For example, if your site doesnt use Oracle then theres no point in using the 'Oracle SQL Injection Enumeration' scan rule.
Another option is to perform regular scans using only the scan rules that you think are the most important, and doing a full scan less frequently.
Which rules are taking the most time?
You can find this out in the zap.log file.
If you post to the ZAP user group then we'll do our best to help you with this.