Another XML-style approach...

drop table #Results

; with a as (select RowNum, cast('<parm '+ replace(replace(Parameters,'=','="'),'&','" ')+ '"/>' as xml) as Xml from #SourceTable)

select RowNum,

Xml.value('(parm[1]/@Subject)[1]', 'varchar(255)') as Subject,

Xml.value('(parm[1]/@Category)[1]', 'varchar(255)') as Category,

Xml.value('(parm[1]/@Status)[1]', 'varchar(255)') as Status,

Xml.value('(parm[1]/@IPPDM_Count)[1]', 'varchar(255)') as IPPDM_Count,

Xml.value('(parm[1]/@Well_Count)[1]', 'varchar(255)') as Well_Count

into #Results

from a

This seems to run much faster for me, but perhaps I'm misunderstanding the problem?