SET STATISTICS IO ON 
SET STATISTICS TIME ON 
GO

SELECT    TOP 50 Tag1.PostId, LastActivityDate
FROM      (
    SELECT p.Id PostId, LastActivityDate
      FROM Posts p
      JOIN PostTags pt ON p.Id = pt.PostId 
     WHERE pt.TagId = 3
    ) Tag1
LEFT JOIN (
    SELECT p.Id PostId
      FROM Posts p
      JOIN PostTags pt ON p.Id = pt.PostId 
     WHERE pt.TagId = 820
    ) Tag2 ON Tag1.PostId = Tag2.PostId
WHERE     Tag2.PostId IS NULL
ORDER BY  LastActivityDate DESC

SET STATISTICS TIME OFF
SET STATISTICS IO OFF
GO

Table 'PostTags'. Scan count 5, logical reads 6344, [...]
Table 'Posts'. Scan count 5, logical reads 302062, [...]
SQL Server Execution Times: CPU time = 5173 ms, elapsed time = 2194 ms. ​

SET STATISTICS IO ON 
SET STATISTICS TIME ON 
GO

SELECT    TOP 10 PostTags.TagId, COUNT(1) as Number
FROM      (
    SELECT p.Id PostId, LastActivityDate
      FROM Posts p
      JOIN PostTags pt ON p.Id = pt.PostId 
     WHERE pt.TagId = 3
    ) Tag1
LEFT JOIN (
    SELECT p.Id PostId
      FROM Posts p
      JOIN PostTags pt ON p.Id = pt.PostId 
     WHERE pt.TagId = 820
    ) Tag2 ON Tag1.PostId = Tag2.PostId
JOIN      PostTags ON PostTags.PostId = Tag1.PostId
WHERE     Tag2.PostId IS NULL
AND       PostTags.TagId NOT IN (3, 820)
GROUP BY  PostTags.TagId
ORDER BY  2 DESC

SET STATISTICS TIME OFF
SET STATISTICS IO OFF
GO

Table 'PostTags'. Scan count 15, logical reads 169532, [...] 
read-ahead reads 122731 [...]
Table 'Posts'. Scan count 10, logical reads 329038, [...] 

SQL Server Execution Times: 
  CPU time = 18875 ms, elapsed time = 5399 ms. 

SET STATISTICS IO ON 
SET STATISTICS TIME ON 
GO

SELECT   TOP 100 Id, LastActivityDate
FROM     Posts 
WHERE    PostTypeId = 1
AND      CONTAINS(Tags, '"javascript" AND NOT "jquery"')
ORDER BY 2 DESC

SET STATISTICS IO OFF
SET STATISTICS TIME OFF 
GO

Table 'Posts'. Scan count 5, logical reads 179483, [...]

SQL Server Execution Times: 
CPU time = 5202 ms, elapsed time = 2044 ms. ​ 

var results = new int[50];
var count = 0;
var relatedTags = new Dictionary<string, int>();

for (var i = 0; i < Index.length /* 10,000,000 */; i++)
{
    if (matcher.Match(Posts[Index[i]]) {
        count++;
        if (count<51) {
            results[count-1] = Posts[Index[i]]; 
        }
    }
    
    for(int t = 0; t<5; t++) {
        relatedTags[Posts[Index[i]].Tags[t]] = 
            relatedTags[Posts[Index[i]].Tags[t]] + 1;
    }
}

public class Matcher {

    public bool NonClosedOnly { get; set; }
    public bool NoAnswerOnly { get; set; }
    public bool BountiesOnly { get; set; }

    /* a few of these */

    public static bool Match(Post) {
        if (NonClosedOnly && Post.DateClosed != null) return false;
        if (NoAnswerOnly && Post.Answers > 0) return false;
        if (BountiesOnly && Post.BountyAmount == 0) return false;

        /** a few of these **/

        return true;
    }
}

public interface IMatcher { public bool Match(Post post) }

public class MatcherGenerator {
    public bool NonClosedOnly { get; set; }
    public bool NoAnswerOnly { get; set; }
    public bool BountiesOnly { get; set; }
    /* a few of these */

    public Func<Post, bool> GenerateMatch() {
        var il = IL.Implement(typeof(IMatcher).GetMethod(nameof(IMatcher.Match)));
        Label exclude = il.DefineLabel();

        if (NonClosedOnly) {
            EmitMemberGet(il, nameof(QuestionValue.IsClosed));
            il.Emit(OpCodes.Brtrue, exclude);
        }
        if (NoAnswersOnly) {
            EmitMemberGet(il, nameof(QuestionValue.AnswerCount));
            il.Emit(OpCodes.Brtrue, exclude);
        }
        if (BountiesOnly) {
            EmitMemberComparison(il, nameof(QuestionValue.BountySize), null);
            il.Emit(OpCodes.Brtrue, exclude);
        }
        /* a few of these */

        il.EmitLabel(exclude); /* etc */
        return il.CreateDelegate();
    }
}