Extreme slow response ticket search in CSV format

[roymoor]

Hello, when I try to search after upgrading OPTRS 2.4.4 via 3.0.11 to 3.1.10, the performance is really bad.
When the result is shown on the screen, the searchspeed is just fine but when I try to search and download in CSV file, it's response is too slow. It takes about 546 seconds before the webserver returns a timeout message.
This only occurs when I search for more than one thousand tickets.
During the search, two types of select statements are seen. One takes 0.2 seconds on average to complete, the other on aboutn 0.6 seconds.
After a while, I get a Apache timeout with an internal error and the search fails.
Can anyone explain why the search is that slow in this new release? Does this has something to do with the newly introduced Dynamic fields?
I cannot find any suggestions on-line on how to solve this issue.
I'm using the exactly same hardware, data and platform for both 2.4.4 as 3.1.10. My platform is Linux and MySQL.

The two queries that are sent to the database are as follows:
SELECT th.name, tht.name, th.create_time FROM ticket_history th, ticket_history_type tht WHERE th.history_type_id = tht.id AND th.ticket_id = '<ticketid>' AND tht.name = 'Lock' ORDER BY th.create_time, th.id ASC LIMIT 100;
SELECT a.create_time,a.id FROM article a, article_sender_type ast, article_type art WHERE a.article_sender_type_id = ast.id AND a.article_type_id = art.id AND a.ticket_id = '<ticketid>' AND ast.name = 'agent' AND (art.name LIKE 'email-ext%' OR art.name LIKE 'note-ext%' OR art.name = 'phone' OR art.name = 'fax' OR art.name = 'sms') ORDER BY a.create_time LIMIT 1

[jojo]

is the apache error log telling something?

did you run a optimize table after migration?