Query that immobilizes the dev server

Please help figure out why query is making server "non-responsive" extreemly slow, have to reboot everytime I run this update stmt that a developer is running in Dev.


--
Facts: # of recs in tables
select * from themls_offices -- 93414
--
SELECT COUNT(*) -- 69684
from [devdb.helios.themls.com].[mls].[dbo].[all_carets_offices]  ACO

--
Is the update below checking/reading 6,557,849,628
(69684 X 70202 recs)??
I say this because it appears to be the case from the query and when run, it brings down the dev server (extreemly slow, non responsive)

update themls_offices
set CARETS_OfficeID = AgentOfficeID
from [devdb.helios.themls.com].[mls].[dbo].[all_carets_offices] as ACO
where firmCode = cast(ACO.MlsOfficeid as varchar(30))
--
--**I replaced above query by one below - is it equivalent? (FirmCode and CARETS_OfficeID  belong to themls_offices ...)
update themls_offices
set CARETS_OfficeID = aco.AgentOfficeID
from [devdb.helios.themls.com].[mls].[dbo].[all_carets_offices]  ACO
inner join themls_offices MO
on MO.firmCode = cast(ACO.MlsOfficeid as varchar(30))
--*
I expected  69684 recs to be update (see below)
However, the query is taking over 30 min to run. still running
(maybe becasue it is going accross servers - linked server ??)

SELECT COUNT(*) -- 69684
from [devdb.helios.themls.com].[mls].[dbo].[all_carets_offices]  ACO
inner join themls_offices MO
on MO.firmCode = cast(ACO.MlsOfficeid as varchar(30))


--*