mysite slow down but admin is quick

When i goto the admin page is quick,but the home page and the product page is so slowly(pcpartsltd.com),sometimes may be 500 error, and the blog is quick(pcpartsltd.com/blog/),what is wrong with my site?

when i use

define('STRICT_ERROR_REPORTING', true);

before if (defined('STRICT_ERROR_REPORTING') && STRICT_ERROR_REPORTING == true)

at application_top.php ,

the home page , list page and product page show many notice

such as

Notice: Constant FILENAME_POPUP_SHIPPING_ESTIMATOR already defined in /home/content/77/6009777/html/includes/filenames.php on line 157

Notice: Constant FILENAME_POPUP_ASK_A_QUESTION already defined in /home/content/77/6009777/html/includes/extra_datafiles/ask_a_question_filenames.php on line 24

Notice: Constant FILENAME_ABOUT_US already defined in /home/content/77/6009777/html/includes/extra_datafiles/contact_us_frame_filenames.php on line 11

Notice: Constant FILENAME_DEFINE_ABOUT_US already defined in /home/content/77/6009777/html/includes/extra_datafiles/contact_us_frame_filenames.php on line 12

Notice: Constant FILENAME_PREV_NEXT already defined in /home/content/77/6009777/html/includes/extra_datafiles/faq_manager_file_names.php on line 36

Notice: Constant FILENAME_PREV_NEXT_DISPLAY already defined in /home/content/77/6009777/html/includes/extra_datafiles/faq_manager_file_names.php on line 37

Notice: Constant FILENAME_HOW_TO_BUY already defined in /home/content/77/6009777/html/includes/extra_datafiles/page_7_filenames.php on line 11

Notice: Constant FILENAME_DEFINE_HOW_TO_BUY already defined in /home/content/77/6009777/html/includes/extra_datafiles/page_7_filenames.php on line 12
..........
otice: Constant CATEGORY_CONTACT already defined in /home/content/77/6009777/html/includes/languages/english/extra_definitions/slucky/links_manager_defines.php on line 29

Notice: Constant PRODUCT_NAME_2717 already defined in /home/content/77/6009777/html/includes/classes/seo.url.php(1062) : eval()'d code on line 1

Notice: Constant PRODUCT_NAME_7 already defined in /home/content/77/6009777/html/includes/classes/seo.url.php(1062) : eval()'d code on line 2

Notice: Constant PRODUCT_NAME_2719 already defined in /home/content/77/6009777/html/includes/classes/seo.url.php(1062) : eval()'d code on line 3
................................
Notice: Trying to get property of non-object in /home/content/77/6009777/html/includes/modules/product_listing.php on line 63

Notice: Undefined variable: product_check in /home/content/77/6009777/html/includes/modules/product_listing.php on line 76

Notice: Trying to get property of non-object in /home/content/77/6009777/html/includes/modules/product_listing.php on line 76

Notice: Undefined variable: product_check in /home/content/77/6009777/html/includes/modules/product_listing.php on line 77
...............

help me !

You've obviously got a large number of plugins added to your site. Any one (or all) of them could be causing you significant performance problems.

I'd start by turning off the "seo url" thing.

And I'd do all of the things recommended in this FAQ article: http://www.zen-cart.com/content.php?63-how-do-i-speed-up-my-site and the related links contained in it.

turning off the "seo url" thing???

if i turn off this plugin,my site's url maybe look so complex,i want it easy,,,,

any suggestion???

Yes. Turn the plugin off and see if your problem goes away. If so, then you know there's something wrong with the plugin and not with Zencart), and can react accordingly.

Hello! As far as I know, it is not just a module slowing down your site. Images, scripts and many other things can cause the slow down too. I've come across a blog post on speeding up the store. Maybe it'll be interesting for you.