It sounds like your host is throttling your bandwidth. WordPress uses admin-ajax all the time as do many other plugins. I doubt Formidable is causing admin-ajax to overload. How many calls to admin-ajax are showing up on the network tab in the browser's developer tools? Your form design could be at fault as well. How are you performing the calculations? In PHP or jQuery? How are you performing the lookups?

in loading the form, filling the minimum fields, doing 1 lookup, making the payment and seeing the confirmation, I have 56 admin-ajax.php request. The lookups are done as follows 1. form with entries called "clubs"6 fields70; name: "ID"; type: number 71; name "Code"; type: text72; name "naam"; type tekst73; name: "Federatie/groep"; type drop down74; name: "club/vereniging"; type: tekst75: name: "korting"; type: number 2. form to be filled in by participantmultiple fields s.a. firstname / lastname / email address (with confirmation); several text fields, data field with calculation; etc. (happy to send you the XML of the form).plenty of fields that are shown depending on previous selections.plenty of fields being filled or calculated based on other fiels.Field 316 is a Dynamic Field, drop down display type.It loads the options from the form entries of the "clubs" formSelect field is "club/verening" and these are sorted A to ZOption autocomplete is on. Hope this helps in helping me...