Website loading very slowly

1) The Website is taking 35-40 seconds to load, checking with the hosting provider, they have pointed that the issue is related to the 49 JavaScript files served from, and has said they should be combined into as few files as possible.

Please advise what should be done to fix this via Asset Optimization.

2) Sometimes the site is throwing 500 error, while browsing through the dashboard pages:

Error 500 - Internal Server Error
An error was encountered while processing your request. Typically this is a temporary condition. Please contact the web site owner for further assistance.

Hosting provider seems to point that it's coming from WPMU DEV plugins.