[SalesForce] show updated fields without reloading Lightning page

I think Santanu has a great suggestion but if you do not want every attempt being saved, and/or I think that this would renew their session (which seems like a pretty big "No No" for security) I would instead suggest trying to use a local solution like accessing the data via localStorage versus an Apex Controller. localStorage is quite secure and SFDC does a great job of preventing XSS so I would think that something simple that maybe loads the values of the fields into localStorage would be a great solution.

<apex:page>
  <apex:form id="form1">
    <apex:inputField id='field1' />
    <apex:inputField id='field2' />
    <apex:commandButton value="save" action="{!save}"/>
  </apex:form>
  <script>
    // Used to store your values
    var result = {}
    // just to help debounce saving everything into localstorage
    var timer
    // This will be placed on all the 
    function onChangeEvent () {
      if (timer) clearTimeout(timer)
      var key = this.event.target.id
      var value = this.event.target.value
      result[key] = value
      timer = setTimeout(function(){
        localStorage.setItem('autoSaved', result)
        clearTimeout(timer)
      }, 1000)
    }
    var ids = [
      '{!$Component.form1.field1}',
      '{!$Component.form1.field2}'
    ]
    function addAllSavedValues () {
      result = localStorage.getItem('autoSaved') || {}
      ids.map(function(id) {
        // If the value existed in local Storage proceed
        if (result && result[id]) {
          // get the field by the ID
          var field = document.getElementById(id)
          // change the value of the field using the previous ID loaded into storage
          field.value = result[id]
        }
        // Adds the change event listener to the field to make sure that you track all changes
        field.addEventListener('change', onChangeEvent)
      })
    }

    // Set a Timeout to make sure that the Dom is ready
    setTimeout(addAllSavedValues, 1000)
  </script>
</apex:page>

This should allow you to keep your records low and not renew sessions just because somebody forgets to leave a page while at the same time allow them to save the information that they were working on and make them happy.

The calls themselves are asynchronous, but actions queued around the same time all go to the server at once, which are then processed in order, as outlined in posts like this one, and mentioned in Queueing of Server-Side Actions. You might want to set some of the actions as background actions to improve performance.

action.setBackground();

Foreground and Background Actions states the following note, though:

Don’t rely on each background action being sent in its own request as that behavior isn’t guaranteed and it can lead to performance issues. Remember that the motivation for background actions is to isolate long-running requests into a separate request to avoid slowing the response for foreground actions.

So, for the API calls you expect to take longer, move them to background actions to allow the smaller components to load faster.