This issue usually occurs with AD tree which has many nested nodes (e.g 450). There is no problem to open "Select Organizational Unit (OU)" picker to select required OUs 1st time, the issue is reproducible only when many OUs were selected and if you click on "starting from" to edit the selection. When "Select Organizational Unit (OU)" is loading, console/ IE is completely stuck.
In any case there is no problem to run AD import rules, the only impact is that selected OUs can't be modified due to very slow picker.
Internet Explorer warning: "Stop running this script? A script on this page is causing your web browser to run slowly. If it continues to run, your computer might become unresponsive."
This issue is targeted to be fixed in ITMS 7.5 SP1 HF3
ITMS 7.5 SP1