Main attributes of MYISAM engine that can affect performance

MySQL & MariaDB Other MySQL® Questions
1

I have a huge table of around 5,000,000 subscribers. The table has one primary key which will be used to perform a search on any user.
What I am wondering is

  1. Will the number of threads connected to Mysql by my application have any performance impact, if yes then what should be the optimum value.
  2. I understand that the Key-Buffer)_size should be 40% of available memory?
  3. I am not using a query cache as i will be updating table as frequently as I will perform a select query.
  4. Which other attributes can be tweaked to get the maximum performance.

I really appreciate any advise.

2

This question is much more complex than the simple and specific things you have asked about. I would recommend reading our book, High Performance MySQL Second Edition.