The time for loading the Overview depends on several factors:
Number of resource calendars in the installation.
The connection to the server
Number of appointments in each calendar.
Outlook performance. Calendar Browser can only read the calendars in the same way as Outlook does, which means that everything you do to improve performance in Outlook also is an improvement for Calendar Browser.
Number of views selected for the Overview in the Calendar Browsersettings, first tab.If the option “Show calendars images in Overview” is selected the loading will take longer.
Generally you could consider to you can break the 25 rooms into smaller units, say per floor or department, and make one installation for each unit. That would increase the performance considerably. As it is now, Calendar Browser has to read all 25 to see what times are included in the chosen period.