> Can you share me some tips to improve performance for select queries in myiasam tables ?
Not that easily -- Give me 100 MyISAM queries; I'll give you 70 tips.
Here are some generic tips:
http://mysql.rjweb.org/doc.php/ricksrots
Novices (don't know if you are such) often need to understand the basics of indexing:
http://mysql.rjweb.org/doc.php/index1
A caution about the /memory link... It may be too generous for a 2GB machine like yours.
> key_buffer = 1024M
is much too big -- That is for MyISAM indexes only. That does not leave much room for _data_. Suggest 200M instead.
> thread_cache_size = 512
--> 5
> query_cache_size = 256M
--> 10M
At 256M (1) You are using too much of your limited RAM, and (2) the query cache becomes inefficient at even that size.
> query_prealloc_size = 65536
> query_alloc_block_size = 131072
Leave them at their defaults.