Speed Suddenly Decreased

Your TTFB is over 5 seconds from my location. Try installing a caching plugin like Cache Enabler, it is a pretty basic one, and only provides static page caching.

Contact your hosting provider and talk to them about HTTP/2.

Edit .htaccess file in your WP root directory, and add headers for browser caching, like this:

# Expires headers (for better cache control)
<IfModule mod_expires.c>
ExpiresActive on
    ExpiresDefault                              "access plus 1 month"
    # cache.appcache needs re-requests in FF 3.6 (thanks Remy ~Introducing HTML5)
    ExpiresByType text/cache-manifest           "access plus 0 seconds"
    # Your document html
    ExpiresByType text/html                     "access plus 0 seconds"
    # Data
    ExpiresByType text/xml                      "access plus 0 seconds"
    ExpiresByType application/xml               "access plus 0 seconds"
    ExpiresByType application/json              "access plus 0 seconds"
    # Feed
    ExpiresByType application/rss+xml           "access plus 1 hour"
    ExpiresByType application/atom+xml          "access plus 1 hour"
    # Favicon (cannot be renamed)
    ExpiresByType image/x-icon                  "access plus 1 week"
    # Media: images, video, audio
    ExpiresByType image/gif                     "access plus 4 months"
    ExpiresByType image/png                     "access plus 4 months"
    ExpiresByType image/jpeg                    "access plus 4 months"
    ExpiresByType image/webp                    "access plus 4 months"
    ExpiresByType video/ogg                     "access plus 4 months"
    ExpiresByType audio/ogg                     "access plus 4 months"
    ExpiresByType video/mp4                     "access plus 4 months"
    ExpiresByType video/webm                    "access plus 4 months"
    ExpiresByType image/avif                    "access plus 4 months"
    ExpiresByType image/avif-sequence           "access plus 4 months"
    # HTC files  (css3pie)
    ExpiresByType text/x-component              "access plus 1 month"
    # Webfonts
    ExpiresByType font/ttf    "access plus 4 months"
    ExpiresByType font/otf    "access plus 4 months"
    ExpiresByType font/woff   "access plus 4 months"
    ExpiresByType font/woff2  "access plus 4 months"
    ExpiresByType image/svg+xml                 "access plus 4 months"
    ExpiresByType application/vnd.ms-fontobject "access plus 1 month"
    # CSS and JavaScript
    ExpiresByType text/css                      "access plus 1 year"
    ExpiresByType application/javascript        "access plus 1 year"
</IfModule>

Source