I can't find the exact reason without check your wordpress but i can give you few steps from which you can increase speed of your wordpress website.
You have put below cache code in your .htaccess file so that cache will be saved at client browser's end result in webpage speed fast.
ExpiresByType image/jpg "access 1 year"
ExpiresByType image/jpeg "access 1 year"
ExpiresByType image/gif "access 1 year"
ExpiresByType image/png "access 1 year"
ExpiresByType text/css "access 1 year"
ExpiresByType text/html "access 1 month"
ExpiresByType application/pdf "access 1 month"
ExpiresByType audio/x-wav "access plus 1 month"
ExpiresByType audio/mpeg "access plus 1 month"
ExpiresByType video/mpeg "access plus 1 month"
ExpiresByType video/mp4 "access plus 1 year"
ExpiresByType video/quicktime "access plus 1 month"
ExpiresByType video/x-ms-wmv "access plus 1 month"
ExpiresByType application/x-shockwave-flash "access 1 month"
ExpiresByType image/x-icon "access 1 year"
ExpiresByType application/x-font-woff "access plus 1 year"
ExpiresDefault "access 1 month"
You can follow below steps to speed up your wordpress website:
1. Update Wordpress
2. Delete inactive plugins to speed up your blog
3. Optimize and resize your images
You could use a plugin such as WP Smushit which automatically reduces image file sizes every time you add an image to a page or post. Recommend that you use a GIF or JPG file
4. Use Cache plugin
Check cache tutorial9
WP Super Cache
W3 Total Cache
5. Use CDN (Content Delivery Network)
A CDN is a way of placing copies of the data from your website on several fast secure servers which respond to requests from the server nearest to the user. This increases the loading speed of your website and can be used in conjunction with a caching plugin.
Free CDN : CloudFlare
6. Important DB Cache Reloaded Fix is a modified version of DB Cache Reloaded, patched for WordPress 3.* compatibility. DB Cache Reloaded Fix caches the MySQL queries performed on your database to optimize the site’s speed.
7. Post revisions and Autosaves
WordPress saves a post revision every time you click Save Draft or Update Post and it will store the last 25 revisions for each post on your blog. It will also perform an autosave every 60 seconds. Unfortunately as standard you cannot change these settings. Imagine all of this extra information in your database and how this will affect the speed of your blog.
There are, however, ways to change this, either by using a plugin or by editing your wp-config.php file. By far the easier is to use a plugin if you are not confident with code.
Use the WP-CMS Post Control plugin which allows you to turn off both the autosave and revisions saved. Alternatively, you can also set how many revisions are saved when amending your content instead.
If you don’t want to use a plugin, you can add the following code to your wp-config.php file:
define(‘AUTOSAVE_INTERVAL’, 120 ); // seconds (default is 60)
define(‘WP_POST_REVISIONS’, false ); // disable post revisions
define(‘WP_POST_REVISIONS’, 3); // alter the number of post revisions kept.
Please make sure you backup your wp-config.php file before you make any amendments. By removing all of this extra data, it will help to speed up your blog.
8. Optimize your homepage
Your homepage is an important part of your site, it’s where people will be landing most often, so speed is key! To help speed up your blog and especially your homepage, you can do the following:
- reduce the number of posts shown on the page to around 5;
- remove sharing widgets – include these only in your posts;
- use the more tag and only show excerpts instead of full posts;
in other words, remove general clutter from your homepage to enhance the visitor’s experience.
9. Optimize your WordPress database
WP-Optimize is a WordPress plugin which allows you to clean up your WordPress database.
10. Check P3 Plugin Performance
With P3, you can check which plugins are slowing down your website and take action.
11. Combining CSS or JS files to reduce HTTP requests and file size.
12. Add LazyLoad to your images
LazyLoad is the process of having only only the images above the fold load (i.e. only the images visible in the visitor’s browser window), then, when reader scrolls down, the other images begin to load, just before they come into view.
13. Clean Options
Clean options is simple plugin that allow you to remove unused options from wp_options file table which will help you to reduce loading time of your site because it helps you to save few MBs or KBs.