The urgency does not seem to be very high, since your reply was about 2 months after my prior response.
When you have such an old server (4.99.8 is 3.5 years old) it is likely that you have just a single connection thread pool. With queries in the range of 3 seconds and a single connection thread pool configured, it is easy to block this pool completely, leading to a queuing situation (which is in the user perception a "hang"). The feature of dynamic connection thread pool mapping was introduced with NaviServer 4.99.15 early last year (see e.g. [1]). With this one can map slow requests dynamically to an own pool, where such requests might pile up, but they don't block other traffic.
My recommendation is to update NaviServer to a recent version and use dynamic thread pool mapping. Concerning exceptions from nsstats: the NaviServer modules are released in concert with matching versions in the *modules* directories (see e.g. [2]). there is some tolerance in backward compatibility, but as it looks not so far.
In case, the mapping is not sufficient, and there are more configuration issues, the newer, more detailed statistics can bring more insights.
[1] https://www.mail-archive.com/naviserver-devel@lists.sourceforge.net/msg03446.html
[2] https://sourceforge.net/projects/naviserver/files/naviserver/4.99.16/
