Optimizing form performance
If you have a very long form that is not performing well, first consider the discussion in Maximum form length. The solution may be to use a newer, more powerful device if your form is very demanding; or, you might choose to split one very long form into several smaller ones.
However, you might also be able to improve the performance of your form by altering some aspects of its design. To begin with, it is helpful to understand the two key reasons why users might encounter sluggishness when filling out a form:
Memory: All form fields, choices on the choices sheet, and translations are stored in memory while a form is being filled out. This includes repeated fields, so if you have a 100-field repeat group that's repeated 20 times, then that's 2,000 fields in memory (even if the fields are not currently relevant and so aren't visible). To protect the OS and other apps, Android and iOS only give a limited amount of memory to each app – and when memory gets low things start getting very sluggish because of how they try to conserve remaining memory.
Expression dependencies: Every reference to another field or group within a relevance, calculation, or constraint expression creates a dependency that is watched for potential changes, so that the appropriate expressions can be re-evaluated whenever something on which they depend might have changed. So if Field A refers to Fields B and C and Field C refers to Field D, then moving forward or backward from Field D will trigger re-evaluations of the expressions for Fields C and then A. If you have 3,000 fields in memory (including all instances of repeated fields), then there will be up to 3,000 relevance, constraint, and calculation expressions that are always being considered for re-evaluation (including the relevance expressions for all of those fields not currently visible). So when you move from one field to the next and you experience a delay, ask yourself: what can it be doing just now? Think about the field that you're moving away from and what might depend on that field's value. Sometimes, with a long, complex form, one field can trigger several other fields to recalculate, and those can trigger several more, and the dependencies run so deep that hundreds or even thousands of expressions will be re-evaluated. Thus, in this way, performance will depend on the logic of your form.
Understanding these causes of sluggishness, there are a few things that you can do to improve performance:
Move choices to pre-loaded .csv files: If you have a large number of choices on your choices worksheet (e.g., over 1,000), you should consider moving longer lists to pre-loaded .csv files. This will move the longer lists out of memory and will, in general, improve performance. See Loading multiple-choice options from pre-loaded data for details.
Reduce the number of repeats: Some form designs involve a large number of repeated groups, most of which are never visible. For example, say that you want to ask a long series of questions about certain assets that might be found in a household; if there are 175 different types of assets, you might create a repeat group that's repeated 175 times, with only the relevant assets visible for any given household. In principle, this is a fine design – but the 175x fields in memory can cause the form to perform poorly. Generally, in these cases, there is an alternative design that achieves the same result without so many fields in memory. (In this example case, you could have a multiple-choice question with 175 choices, then repeat the list of questions just once for each selection made by the user. That way, there are only as many fields as necessary to cover the assets in any given household.)
Reduce the number of dependencies: If you've designed your form such that long, complex expressions create such intricate dependencies between fields that any change to one field results in re-evaluation of dozens or hundreds of complex down-stream expressions, the solution may be to simplify your form. However, you should be able to take full advantage of the electronic medium to constrain user entries, catch potential errors, etc. – so in some cases the solution may be to deploy your survey on more powerful devices that can handle whatever logic your survey requires.
Neither Android, nor iOS, nor SurveyCTO is infinitely powerful, so there will always be limits to what can perform well. But devices grow more powerful by the day, and we at SurveyCTO are always pushing the performance frontier outward. If you are using an older version of SurveyCTO, you should try upgrading to the latest version to see if that performs better. We try to include performance improvements whenever we can.