I have an other index on the table prod_starts:
CREATE NONCLUSTERED INDEX [FK_prod_starts_fitgrp_id] ON [dbo].[prod_starts]
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
This morning I discovered the select is using this. I try to disable this index and it works well, with the right index and very speed.
I don't understand why Sql Server chooses that index.
I'll wait for a week with the index disabled, I hope it changes its statistics and fixes the situation