Just wanted to add to ROFL's answer. I have no love at all for CTE's. They are almost always slower than using a temporary table or selecting from a subquery.

CTE's are usually slower than Table Variables too, in my experience with SQL Server at least, although there is little reason to use a Table Variable if you have more than a handful of rows.

I took the liberty of taking the code from ROFL's answer and changing the CTE into a subquery that is directly selected from, it significantly improved the speed, down from ~60ms to ~30ms.

Here is my alteration:

declare @Language nvarchar(25) = ##LanguageTag:string##;
declare @tagid int;
declare @epoch datetime = DateDiff(yy, -68, current_timestamp);

select @tagid = Id
from Tags
where TagName = @Language

print @Language + '->' + Convert(NVarchar(max), @tagid);

Select CreationMonth, 
count(*) as [Count of Tickets], 
avg(Elapsed)/3600.0 as [Average hours to first Answer]
From 
(
 Select Q.Id,
 Convert(Date, DateAdd(day, 1 - DatePart(day, Q.CreationDate), Q.CreationDate)) as CreationMonth,
 Convert(float, DATEDIFF(second, Q.CreationDate, min(A.CreationDate))) AS Elapsed
 From PostTags T,
 Posts Q,
 Posts A 
 Where T.TagId = @tagid
 and T.PostId = Q.Id
 and Q.PostTypeId = 1
 and A.PostTypeId = 2
 and Q.ClosedDate is null
 and Q.Id = A.ParentId
 and A.CreationDate > Q.CreationDate -- validation, and also merges.
 and Q.CreationDate >= @epoch
 and A.CreationDate < CURRENT_TIMESTAMP
 Group by Q.Id,
 Q.CreationDate,
 Convert(Date, DateAdd(day, 1 - DatePart(day, Q.CreationDate), Q.CreationDate))
) As AnswerTime
Group by CreationMonth
order by CreationMonth

And here is it working  

I just wanted to add to @rolfl's answer. I have no love at all for CTEs. They are almost always slower than using a temporary table or selecting from a subquery.

CTEs are usually slower than Table Variables too, in my experience with SQL Server at least, although there is little reason to use a Table Variable if you have more than a handful of rows.

I took the liberty of taking the code from ROFL's answer and changing the CTE into a subquery that is directly selected from, it significantly improved the speed, down from ~60ms to ~30ms.

Here is my alteration:

declare @Language nvarchar(25) = ##LanguageTag:string##;
declare @tagid int;
declare @epoch datetime = DateDiff(yy, -68, current_timestamp);

select @tagid = Id
from Tags
where TagName = @Language

print @Language + '->' + Convert(NVarchar(max), @tagid);

Select CreationMonth, 
count(*) as [Count of Tickets], 
avg(Elapsed)/3600.0 as [Average hours to first Answer]
From 
(
 Select Q.Id,
 Convert(Date, DateAdd(day, 1 - DatePart(day, Q.CreationDate), Q.CreationDate)) as CreationMonth,
 Convert(float, DATEDIFF(second, Q.CreationDate, min(A.CreationDate))) AS Elapsed
 From PostTags T,
 Posts Q,
 Posts A 
 Where T.TagId = @tagid
 and T.PostId = Q.Id
 and Q.PostTypeId = 1
 and A.PostTypeId = 2
 and Q.ClosedDate is null
 and Q.Id = A.ParentId
 and A.CreationDate > Q.CreationDate -- validation, and also merges.
 and Q.CreationDate >= @epoch
 and A.CreationDate < CURRENT_TIMESTAMP
 Group by Q.Id,
 Q.CreationDate,
 Convert(Date, DateAdd(day, 1 - DatePart(day, Q.CreationDate), Q.CreationDate))
) As AnswerTime
Group by CreationMonth
order by CreationMonth

And here is it working