You can define your column like this:
performance tinyint not null check (performance in (0, 1, 2))
tinyint takes only 1 byte for a value and values can range from 0 to 255.
If you store the values as 1 - Low, 2 - Medium, 3 - High and are using SQL server 2012+, then you can simply use CHOOSE
function to convert the value to text when select like this:
select choose(performance,'Low','Medium','High')
. . .
If you really want to store as 0,1,2, use :
select choose(performance+1,'Low','Medium','High')
. . .
If you are using a lower version of SQL server, you can use CASE
like this:
case performance
when 0 then 'Low'
when 1 then 'Medium'
when 2 then 'High'
end