InfoPath Dev

Hi all,
I'm having some performance issues with a big InfoPath form with a repeating section and SharePoint 2013 online. My idea is that the problem might be that there are several drop-down fields which fetch data from other lists (with a data connection), data are also filtered by the choice made in previous fields.

Sometimes I also get the "Critical error: This session has exceeded the amount of allowable resources", but I should be able to solve it by increasing the "Number of Actions per Postback" and "Maximum size of form session state" thresholds.

Anyway, the form is very very slow, it shows the message "sending data to the server..." very often.

Are there any best practices which I may use to improve my form performance?

P.S. I'm uising InfoPath 2010 and Sharepoint 2013 online (I cannot use Sharepoint designer or qRules).

 After doing some research (and since I am not allowed to change the "Number of Actions per Postback" and "Maximum size of form session state" values), I think a solution might be to change my Sharepoint list data connection into a REST connection.

So, I created a REST connection to the list which has the data I need to populate variuos drop-down menus.

Now, I want to fetch and filter data for various drop-down menus based on the choice made in the previuous drop-down menus.

This is  what I made:

1. I created an Action Rule on my first Dropdown menu that Change REST URL to https://mysite.com/sites/myteamsite/_vti_bin/ListData.svc/mycooldatalist?$filter=/dfs:myFields/dfs:dataFields/my:SharePointListItem_RW/my:Dropdown1 and then Query for data using the REST connection;
2. I set the second Dropdown menu to "get choices from an external source" and I choosed as data source my REST connection.

Doing so iteratively, I should be able to have various cascading dropdown menus, each one is based on the choice made of the previous one.

Well... this is not working, I fear I'm doing something wrong on step 1. because I get the error: "Could not open: https://mysite.com/sites/myteamsite/_vti_bin/ListData.svc/mycooldatalist?$filter=/dfs:myFields/dfs:dataFields/my:SharePointListItem_RW/my:Dropdown1"

Any ideas?