In the meantime we have developed a quick fix which might come in handy for people who also experience this issue.

The issue

Using DotTrace we analysed the following snapshot:

We see that the ParsePlaceHolders is being requested a lot (in the Build(Form) method in 4 foreach loops). And the Membership.GetUser is requested foreach ParsePlaceHolder.

public static string ParsePlaceHolders(HttpContext Context, Umbraco.Forms.Core.Record record, string value)
{
  string str = StringHelper.(DictionaryHelper.GetText(value), Context);
  if (record != null)
    str = StringHelper.(str, record);
  if (Member.InUmbracoMemberMode())
  {
    HttpContext.Current.Trace.Write("In member mode");
    str = record == null || record.MemberKey == null ? (Context == null || Context.Session == null || Context.Session["ContourMemberKey"] == null ? (Context == null || Context.User == null || !Context.User.Identity.IsAuthenticated || Membership.GetUser() == null ? StringHelper.(str, (object) 0) : StringHelper.(str, Membership.GetUser().ProviderUserKey)) : StringHelper.(str, Context.Session["ContourMemberKey"])) : StringHelper.(str, (object) record.MemberKey);
  }
  return str;
}

The Quick fix

Create a new class that inherits MembersMembershipProvider and overrides the GetUser(string, bool) method.

public class CustomMembersMembershipProvider : MembersMembershipProvider
{
    public override System.Web.Security.MembershipUser GetUser(string username, bool userIsOnline)
    {
        //get the user from HttpContext Items if available.
        if (HttpContext.Current.Items.Contains(username))
        {
            return HttpContext.Current.Items[username] as System.Web.Security.MembershipUser;
        }

        var user = base.GetUser(username, userIsOnline);
        //Only cache if the authenticated user is equal to the user that needs to be received. (prevents misuse of HttpContext Items)
        if (HttpContext.Current.User.Identity.Name == username)
        {
            //we cache the user in the HttpContext Items to prevent roundtripping to the database.
            HttpContext.Current.Items.Add(username, user);
        }
        return user;
    }
}

Also, override the existing configuration setting for UmbracoMembershipProvider in the web.config /configuration/system.web/membership/providers to use your class (change the namespace and library name):

<add name="UmbracoMembershipProvider" type="[YourLibrary.NameSpace].CustomMembersMembershipProvider, [YourLibrary]" minRequiredNonalphanumericCharacters="0" minRequiredPasswordLength="4" useLegacyEncoding="true" enablePasswordRetrieval="false" enablePasswordReset="true" requiresQuestionAndAnswer="false" defaultMemberTypeAlias="Member" passwordFormat="Hashed"  />