Is it normal for F5 LTM CPU to hike 10% extra with iRule implementation?

You can save a bit of CPU cycles by not doing the data group lookups on each HTTP request. You could do this once at config load time by moving the lookups to RULE_INIT:

when RULE_INIT {
    set static::maxActiveClients "[class search -all -value datagroup_selfridges_waitingroom starts_with "max_active_clients"]"
    set static::subtableName "sessionLimit"
    set static::sessionCookieName "[class search -all -value datagroup_selfridges_waitingroom starts_with "session_cookie_prefix"]"
    set static::holdingPage "[class search -all -value datagroup_selfridges_waitingroom starts_with "holding_page_url"]"
    set static::sessionTimeout "[class search -all -value datagroup_selfridges_waitingroom starts_with "session_timeout"]"
}

If you want some/all of these lookups done more frequently, you could move them to the CLIENT_ACCEPTED event instead. Maybe the timeout and max_active_clients?

You can check if the cookie value isn't null in one step like this:

Current:

if {[HTTP::cookie exists $sessionCookieName]}{

Suggested:

if {[set client_id [HTTP::cookie $sessionCookieName]] ne ""}{

You can save a bit of CPU by using ne instead of != for text comparisons:

Current:

if { $sessiondata != "" } {

Suggested:

if { $sessiondata ne "" } {

Aaron