My Site is loading very slow

Please what can i do to improve the loading speed of my site (Blogspace.pro) and 'm on VPS server and my other websites i have on this same server are doing very well, at times if i open the page or any link on the page it display an error so i have to refresh like 2 or 3 times before the page open this are the kind of error it display

Fatal error: Maximum execution time of 30 seconds exceeded in /home/blogspac/public_html/wp-includes/wp-db.php on line 926
Fatal error: Maximum execution time of 30 seconds exceeded in /home/blogspac/public_html/wp-includes/wp-db.php on line 1549
Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 24 bytes) in /home/blogspac/public_html/wp-includes/wp-db.php on line 1555
Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 32602 bytes) in /home/blogspac/public_html/wp-includes/wp-db.php on line 1558 >