here are some tips for you,
1.Add index on userid and username (grouped index) if userid is not unique/primary key that will improve performance for sure
2.if possible make userid autogenerated and primary key,if you do that your query will reduce to
"select * from user where user_id=1000" which will faster with single index (userid which is default as its a primary key) and that will save space compared to grouped index
3.basic rule for perfomance improvement, always fetch required columns only (select * from is worst always)
do not forget to run runstats and reorg after index creation
clustered index mentioned above will also help but more space is required for clustered index.(why waste more space if your query could be improved or reduced in your case)
hope this helps..