I am submitting a form with a lot of fields and trying to validate it with
// Normalize data to unified representation
$normData = $this->viewToNorm($viewData);
$value = $transformers[$i]->reverseTransform($value);
This question is a little vague, and the answer very much depends on your specific form. Please post the form definition that is giving you the hardest time.
Check to make sure that you are not EAGER fetching associations here.
handleRequest() is going to take the request object and construct the model that your form describes, as your form defined it to.
If the objects are required in order to display data on your initial form to the user, or to validate the data on submit, the "entity" field type will fetch all of the objects you told it to in its definition. If you are displaying a big select list, for example, all of this data is needed.
I had a similar problem in the past and it was because I was using a lot of choice fields that were being used as a series of multiple select checkboxes. My bottleneck was actually in the twig layer while rendering out the thousands of checkboxes I had stored as separate entities.
I switched from a set of checkboxes to a single multi-select box and it increased my speed significantly.