Not sure what do you precisely mean with "IMAP syncing is slow", as in when you tell it to sync it's slower than Thunderbird? Or that you're being notified by Kmail about new mails later than you did with Thunderbird?
Because if it's the latter, you may want to check under Settings -> Accounts -> Reception (or however that tab is called in english) -> Your IMAP account -> General tab -> Sync interval.