Sending message to non-respondents became extremely slow in 3.5

After upgrade to 3.5 sending message to non-respondents is extremely slow.

Test:

• go to non-respondents page
• select one student
• type subject and message
• press send - it is done almost immediately

Test 2:

• go to non-respondents page
• select three students
• type subject and message
• press send - it takes a lot of time to wait for success message

When you select more students, it may timeout waiting for success message. But if you look at maillog - all messages are sent fast, it only waits for something in the end.

This behaviour is seen after upgrade to 3.5