My initial select is like this:

UPDATE Client
 SET DayBetweenFirstAcceptSoldDate=
  CASE
   WHEN Client.FirstAcceptDate>'1899-12-30 00:00:00.000' AND
    Client.QueryDate>'1899-12-30 00:00:00.000' AND
    Client.QueryDate>=Client.FirstAcceptDate AND
    (Client.NoCompany>0 OR Client.NoResearch>0) THEN
    (SELECT COALESCE(SUM(Temp.[Second])/3600.0/8,0)
    FROM (SELECT DATEDIFF(SECOND,
     CASE WHEN BusinessTime.Start>Client.FirstAcceptDate THEN BusinessTime.Start ELSE Client.FirstAcceptDate END,
     CASE WHEN BusinessTime.[End]<Client.QueryDate THEN BusinessTime.[End] ELSE Client.QueryDate END) AS [Second]
     FROM BusinessTime
     WHERE BusinessTime.NoProvince=
       (SELECT COALESCE((SELECT Company.NoProvince FROM Company WHERE Company.NoCountry=1 AND Company.Numero=Client.NoCompany),84))
      BusinessTime.NoHoliday=0 AND
      BusinessTime.Start<Client.QueryDate AND
      BusinessTime.[End]>=Client.FirstAcceptDate) Temp)
   ELSE 0
  END

Now, I need to condition the WHERE clause to support one way or another:

UPDATE Client
 SET DayBetweenFirstAcceptSoldDate=
  CASE
   WHEN Client.FirstAcceptDate>'1899-12-30 00:00:00.000' AND
    Client.QueryDate>'1899-12-30 00:00:00.000' AND
    Client.QueryDate>=Client.FirstAcceptDate AND
    (Client.NoCompany>0 OR Client.NoResearch>0) THEN
    (SELECT COALESCE(SUM(Temp.[Second])/3600.0/8,0)
    FROM (SELECT DATEDIFF(SECOND,
     CASE WHEN BusinessTime.Start>Client.FirstAcceptDate THEN BusinessTime.Start ELSE Client.FirstAcceptDate END,
     CASE WHEN BusinessTime.[End]<Client.QueryDate THEN BusinessTime.[End] ELSE Client.QueryDate END) AS [Second]
     FROM BusinessTime
     WHERE BusinessTime.NoProvince=
      CASE
       WHEN Client.NoCompany>0 THEN
        (SELECT COALESCE((SELECT Company.NoProvince FROM Company WHERE Company.NoCountry=1 AND Company.Numero=Client.NoCompany),84))
       ELSE
        (SELECT COALESCE((SELECT Research.NoProvince FROM Research WHERE Research.NoCountry=1 AND Research.Numero=Client.NoResearch),84))
       END AND 
      BusinessTime.NoHoliday=0 AND
      BusinessTime.Start<Client.QueryDate AND
      BusinessTime.[End]>=Client.FirstAcceptDate) Temp)
   ELSE 0
  END

However, this is having some weird results on SQL Server. I can execute that command, which used to run for about 3 minutes, and now it can takes up to 24 hours and still not finished.

The Client table has 3 million records and all related indexes are there.

I assume the syntax I am using is not a proper approach.

Anyone would know what it could be?