SilverStripe mod_pagespeed install

.htaccess

<IfModule pagespeed_module>
	ModPagespeed on
	ModPagespeedDisallow "*/admin/*"
	AddOutputFilterByType MOD_PAGESPEED_OUTPUT_FILTER text/html
	ModPagespeedLowercaseHtmlNames on
	ModPagespeedEnableFilters prioritize_critical_css,defer_javascript
	ModPagespeedEnableFilters convert_jpeg_to_webp,inline_preview_images
	ModPagespeedEnableFilters collapse_whitespace,remove_comments
</IfModule>

config.yml

# PageSpeed requires no HTTP transform restrictions:
HTTP:
  cache_control:
    no-transform: "false"
    no-store: "false"

pagespeed.sh

#!/usr/bin/env bash

echo "Installing mod_pagespeed"

sudo yum install -y at  # if you do not already have 'at' installed
cd /tmp
wget https://dl-ssl.google.com/dl/linux/direct/mod-pagespeed-stable_current_x86_64.rpm
sudo rpm -U mod-pagespeed-stable_current_x86_64.rpm

Was facing the same issue. It's about the .htaccess configuration within the "assets/" folder.

Use this .htaccess configuration:

AddHandler default-handler php phtml php3 php4 php5 inc

<IfModule mod_php5.c>
        php_flag engine off
</IfModule>

RewriteCond %{REQUEST_FILENAME} !\.(webp|html|htm|xhtml|js|css|bmp|png|gif|jpg|jpeg|ico|pcx|tif|tiff|au|mid|midi|mpa|mp3|ogg|m4a|ra|wma|wav|cda|avi|mpg|mpeg|asf|wmv|m4v|mov|mkv|mp4|ogv|webm|swf|flv|ram|rm|doc|docx|dotx|dotm|txt|rtf|xls|xlsx|xltx|xltm|pages|ppt|pptx|potx|potm|pps|csv|cab|arj|tar|zip|zipx|sit|sitx|gz|tgz|bz2|ace|arc|pkg|dmg|hqx|jar|xml|pdf|gpx|kml)$ [NC,OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule .* / [F]

More information: apache/incubator-pagespeed-mod#1531