Okay I have answered this myself and figured out a way to speed things up.

Posting here in case its useful to anyone.

So in SQL we can use "OFFSET x ROWS FETCH NEXT y ROWS ONLY;"

--page 1

select *
from vCommunication
where cmli_comm_companyid=43 and comm_private is null
order by comm_datetime desc
OFFSET 0 ROWS FETCH NEXT 5 ROWS ONLY;
go

--page 2

select *
from vCommunication
where cmli_comm_companyid=43 and comm_private is null
order by comm_datetime desc
OFFSET 5 ROWS FETCH NEXT 5 ROWS ONLY;

In CRM's api we can append this to the OrderBy property

var x=CRM.FindRecord("communication,vcommunication",
  "cmli_comm_companyid=43 and comm_private is null");

x.OrderBy="comm_datetime OFFSET 0 ROWS FETCH NEXT 3 ROWS ONLY;";

Now you do need to pass up the page and number of rows from your app to build out the correct OFFSET statement

AFTER SOME MORE TESTING THIS BIT BELOW IS WRONG

And also...to get the correct recordcount you in fact need to do this...

var x=CRM.FindRecord("communication,vcommunication",
  "cmli_comm_companyid=43 and comm_private is null");

x.OrderBy="comm_datetime";

var _recordcount=q.recordcount;

x.OrderBy="comm_datetime OFFSET 0 ROWS FETCH NEXT 3 ROWS ONLY;";

...by setting

x.OrderBy

again the query is rerun and only returns 3 rows (in this example)

SO IN FACT WHAT YOU NEED TO DO IS WRITE A QUERY TO GET THIS VALUE....THE TRADE OFF IS THAT THIS MIGHT BE A HIGHER NUMBER THAN THE USER CAN IN FACT SEE...I THINK ITS WORTH IT MYSELF

var _countwry="select count(comm_communicationid) as 'count' from communication where cmli_comm_companyid=43 and comm_private is null";
var _countwryq=CRM.CreateQueryObj(_countwry);
_countwryq.SelectSQL();
var recordcount = _countwryq('count');