You'll notice that the second time you go into the Checkout form, it loads quite quickly.  This is because the form is "cached".

The default timeout for the cache is 10 minutes.

There is a field on the Merchant Facility called "Cache Time".  This may  not be on the page layout and your Salesforce Administrator may need to add it.  Please consult Salesforce online guides, trailhead or Salesforce support if you need assistance adding fields to a Page Layout.

Enter a value into the field "Cache Time" that is higher, such as 2880 for 2 days (in  minutes).

As a side note, we use Salesforce servers to host the forms, so are a little bound by their speed in serving up the forms.

Our forms do have a large amount of security checks built into  them.  Some of these can slow the form down.  Having a secure form is  important for all organisations and the trust of their donors.

We are looking to use a salesforce speed improvement tool called  platform cache.  This is on the development roadmap, but no fixed date  is set yet.

If all of the above does not meet your needs, you can work with  your web developer and create lead in pages, with our form being just say the  credit card entry section.  See:  https://help.payments2us.com/m/userguide/l/1146857-how-to-create-a-custom-lead-in-page