You are welcome, It’s my pleasure to help you
For the website, I logged in successfully. At first the problem was happening yes. I visited the Settings -> Permalinks and just saved the settings without changing them. Sometimes WordPress needs to re-consider the pages links. the problem disappeared now as I see. I created a new page and added it to the menu and made sure it works. I removed all of this no worries.
Please make sure that the 404 issue is resolved now.
For the website speed, I guess it’s a hosting problem. It’s working normally on our demo and other demos we see from our clients. Maybe you’ll need to upgrade the hosting plan or pick another host. There are many WordPress hosts that are both powerful and with good prices. Mediatemple and WPengine are good ones.
Let me know if you need anything further.