Nested host groups can cause 4 to 5 second delay between views in the GUI if you have a lot of nested host groups. Parent/child relationships are not being traversed right causing Zabbix to look up invalid group names.

For example if you have the following host groups:
OS
OS / Windows
OS / Windows / Windows 2012
OS / LInux
...

Zabbix will search the following groups:
OS
OS / Windows
OS / Windows / OS
OS . Windows / OS / Windows 2012
...etc

It is adding the parent group after the child group. This can be seen in the debug view as shown below

2. hostgroup.get [CPageFilter.php:413]

Parameters:
Array
(
[output] => Array
(
[0] => groupid
[1] => name
)

[filter] => Array
(
[name] => Array
(
[0] => OS
[1] => OS / Windows
[2] => OS / Windows /OS
.....etc.