Performance problems are best tackled by first gathering evidence and then using that evidence to identify the root cause and so hopefully the fix.
There are 3 tools I suggest you use and in this order:
- the Network tab of your browsers Developer Tools that presents how long it takes each piece of information to arrive in the browser and the sequence they arrive
- the Developer Console's timeline feature that organises the server debug log timestamps into overview tables and shows the breakout between the database, Apex controller code and Visualforce
- the Developer Console's query plan tool that lets you check how the queries are being executed on the server
Start by running the tools in production and run them in the sandbox as needed to identify the differences.