Recurring Error

We're using Matrix 3.20.1 to run our intranet, which has a lot of Asset Listing pages and lots of documents. Now this install has always been running very very slow and we havent been able to identify why. We do get this one recurring error coming up everytime someone access the intranet


(/core/include/mysource.inc:402) - Undefined index: HTTP_HOST



We also get a Cron Manager error



(/packages/cms/page_templates/page_asset_listing/page_asset_listing.inc:132) - Only variables should be assigned by reference.



Not sure what these means and how to address them. Any help would be most welcome