My first guess is that there could be a slow query somewhere, but i have no idea which.
Can you please turn Joomla debug on (in Joomla general configuration, 2nd tab), enable module, then go to admin homepage and look under SQL queries accordion: It will give you the timing bargraph of all queries, and you can click on the longest (red) bar, If the query is much longer than any other query, you can do a screenshot of it, together with execution time and explain statement below it. Then attach it to your reply here, and we can take a look.