long contact list, slow autocomplete

I’d like to have a rapid (i.e. immediate) autocomplete when I begin typing a name or email.  Instead, I get a super slow pop-up of a huge list that has the string of typed letters somewhere in it, since it is searching rather than completing.  The huge list comes about from having an institutional directory attached to my gmail account, which I cannot delete or ignore.  If I delete the folder “directory” from eM client, it simply repopulates it again as it syncs with gmail.  Any suggestions?  This is making my email almost unusable.