Queries :: More Than 1 Inner Join Slows Query Too Much

I have my main linked table VI, and 4 of VI's fields are to be filtered in a query.

To do so, I have 4 very very tiny local tables (less than 20 records) called respectively "soff", "skus", "warr" and "typ", in which the user will only put what he wants in the result, and by doing Inner Joins, I filter it easily.

Problem is, doing more than 1 Inner Join slow the query so slow it's unbelievable.

If I run the query:

Code:
SELECT VI.*
FROM VI INNER JOIN SKUS ON VI.Sku= SKUS.Sku;

It runs in 0.7 seconds. It's about the same if I replace SKU by SOFF, WARR, or TYP, the query takes about 0.5 to 1.5 sec.

Now if I run:

SELECT VI.*
FROM (VI INNER JOIN SKUS ON VI.Sku= SKUS.Sku)
INNER JOIN SOFF ON VI.SOff = SOFF.Soff;

It takes either 15 seconds, or up to 100 seconds.

I tried by doing 4 successive queries, it's the same. The 1st query runs well, and then it grinds to a near-halt.

Running either the 4 queries or a query with the 4 inner joins takes me about 200 seconds, sometimes 400-500.

It's not even a hard query, VI has only almost 1mil rows, and the fields are indexed. The result is 800 rows.

If each join on VI takes 1 second, it should do 1sec+1sec+1sec+1sec, taking in account the fact that the left side of the join grows smaller at each step, it should even do something like 1+0.8+0.5+0.2, or something. Why does having these joins together, or follow each other, make things so damn slow?