Help with iRule to route by host header to pool

Looks good to me, there is very little to optimize here.

Reading directly from a variable like [HTTP::host] is faster than setting the value in a variable and read that variable after. However, in this case, you are also using the string tolower twice, so not sure if there will be any improvement.

when HTTP_REQUEST {
 test_dg is a string data-group where the entries are name:value pairs
 name is the requested HTTP host header, value is the associated pool-name

     check the requested HTTP host header against entries in data-group
    set host [string tolower [HTTP::host]]
    if { [class match $host equals test_dg ] } {
         if the HTTP host header is in datagroup
         send the request to the pool associated with the datagroup entry
        pool [class match -value $host equals test_dg ]
    } else {
         drop the request if the host header is not in datagroup
        drop
}
}

Have a look in this link about testing the speed of the iRule.

https://devcentral.f5.com/articles/irules-optimization-101-05-evaluating-irule-performance

If you want to read the full article:

https://devcentral.f5.com/articles/irules-optimization-101-01-if-elseif-and-switch