poorly performing URI rule

Since you are just looking for the first characters in the string, have you tried the "starts_with" operator? substr actually allocates another string which has some overhead.

if (tolower(http_uri) starts_with "/ab/") {
  use pool WebPool1
} else if (tolower(http_uri) starts_with "/cdefg/") {
  use pool WebPool1
} else {
  use pool WebPool2
}

Or, you could combine the first two checks into the first if

if ( (tolower(http_uri) starts_with "/ab") or 
     (tolower(http_uri) starts_with "/cdefg/") ) {
  use pool WebPool1
} else {
  use pool WebPool2
}

Those should both perform roughly the same though.

I'm not sure you could optimize this rule more that this.

-Joe