Why are the performances of these 2 queries so different?

OR can crush performance, so do it this way:

DECLARE @Filter VARCHAR(100)SET @Filter = 'FORMSOF(INFLECTIONAL, robe)'IF @Filter IS NOT NULLBEGIN    SELECT TOP 100 ID FROM dbo.Products    WHERE CONTAINS(Name, @Filter)ENDELSEBEGIN    SELECT TOP 100 ID FROM dbo.ProductsEND

Look at this article: Dynamic Search Conditions in T-SQL by Erland Sommarskog and this question: SQL Server 2008 - Conditional Query.

The first query plan looks straightforward:

1. a full text search to resolve CONTAINS(Name, @Filter)
2. an index scan to look up the other columns of the matched rows
3. combine the two using a hash join

The concatenation operator forms a union of two recordsets. So it looks like the second query is doing:

1. an index scan (later used to look up other columns)
2. a constant scan. I assume it's treating your query as not parameterized, so the query plan doesn't have to work for any other value of @Filter. If correct, the constant scan resolves @Filter is not null.
3. a full text search to resolve CONTAINS(Name, @Filter)
4. unions the result of 3 with the empty set from 2
5. loop joins the result of 1 and 4 to look up the other columns

A hash join trades memory for speed; if your system has enough memory, it's much faster than a loop join. This can easily explan a 10-100x slowdown.

One fix is to use two distinct queries:

if @Filter is null    SELECT TOP 100 ID FROM dbo.Productselse    SELECT TOP 100 ID FROM dbo.Products WHERE CONTAINS(Name, @Filter)

You've introduced an OR condition.In most cases it is simply much faster to check explicitly for NULL and perform one query vs your method.

For instance try this:

IF @Filter IS NULL BEGINSELECT TOP 100 ID FROM dbo.ProductsENDELSEBEGINSELECT TOP 100 ID FROM dbo.ProductsWHERE @Filter CONTAINS(Name, @Filter)END