If a HubSpot-hosted website won't load, try accessing the site on a mobile device with Wi-Fi turned off. If the website loads successfully on the mobile device, the issue is likely based in the network used to access it. In this case, visitors not using that network should be able to access the website without issue.
What is wrong with HubSpot?
It's not flexible enough
One of the biggest gripes about HubSpot CRM is that it isn't as customisable as platforms like SalesForce or Microsoft Dynamics. Depending on the complexity of your business, this means it might not be the perfect fit for you.
Why is HubSpot so slow?
One thing you can try is accessing your account in an incognito window and another browser. If the issue isn't present in incognito, then clearing your browser's cache & cookies (instructions here) to ensure that you are accessing the most recent version of your account may resolve the slowness you've been seeing.