I ran mine and Richard's functions on Jeffs test data (pushed up to 1000000 rows) and my function above (at 00:01:25.603) just pipped Richards (at 00:01:57.740).

alter table #HexTest add ASCIIValue varchar (20)

go

if object_id ('tempdb.dbo.#TimeTrial') is not null drop table #TimeTrial

create table #TimeTrial (Event varchar (20), DateStamp datetime default getdate ())

update #HexTest set ASCIIValue = null       -- reset

insert #TimeTrial (Event) values ('RichardStart')

update #HexTest set ASCIIValue = dbo.f_hextostr (HexValue)

insert #TimeTrial (Event) values ('RichardFinish')

select * from #HexTest

update #HexTest set ASCIIValue = null       -- reset

insert #TimeTrial (Event) values ('EddieStart')

update #HexTest set ASCIIValue = dbo.fn_HexToAlpha (HexValue)

insert #TimeTrial (Event) values ('EddieFinish')

select * from #HexTest

select (select DateStamp from #TimeTrial where Event = 'RichardFinish') - (select DateStamp from #TimeTrial where Event = 'RichardStart') Richard

select (select DateStamp from #TimeTrial where Event = 'EddieFinish') - (select DateStamp from #TimeTrial where Event = 'EddieStart') Eddie

Eddie.