Describe the form evaluation process and what kind of expressions are resource-intensive

See https://forum.opendatakit.org/t/odk-collect-crashing-on-tecno-sas6-after-only-one-or-two-interviews/23357 for an example of a user getting an Android Not Responding error with a form that has many calculations in repeats.

Start by reading the linked forum report about ODK Collect becoming unresponsive with forms containing many calculations in repeats. Document how form evaluation works and identify which kinds of expressions are resource-intensive, with completion marked by clear guidance addressing this example and the requested performance concerns.

Written by the indexing model from the issue text.