[exim] exim processes with high CPU load

Hello,

for the two past week or so I noticed an increasing CPU load on our mail
server. When watching the output of "top", there are always some
exim-processes with CPU 17%, 23%, 28% and of course different values in
between. While the most exim-processes are using less than 1%, there are
3-5 processes with changing pid that are using more than 10%. I tried to
log what these processes are doing by running exiwhat in an endless
loop. Often the processes are gone faster than exiwhat can return the
output. I was successful in at least one process, where exiwhat returned
the following lines:

6001 handling incoming connection from p54bc4e87.dip.t-dialin.net
(yuhvlkbv.net) [84.188.78.135]
6001 handling incoming connection from p54bc4e87.dip.t-dialin.net
(yuhvlkbv.net) [84.188.78.135]
6001 handling incoming connection from p54bc4e87.dip.t-dialin.net
(yuhvlkbv.net) [84.188.78.135]
6001 handling incoming connection from p54bc4e87.dip.t-dialin.net
(yuhvlkbv.net) [84.188.78.135]
6001 handling incoming connection from p54bc4e87.dip.t-dialin.net
(yuhvlkbv.net) [84.188.78.135]
6001 handling incoming connection from p54bc4e87.dip.t-dialin.net
(yuhvlkbv.net) [84.188.78.135]
6001 handling incoming connection from p54bc4e87.dip.t-dialin.net
(yuhvlkbv.net) [84.188.78.135]
6001 handling incoming connection from p54bc4e87.dip.t-dialin.net
(yuhvlkbv.net) [84.188.78.135] id=1Eevjx-0001Yn-K5
6001 handling incoming connection from p54bc4e87.dip.t-dialin.net
(yuhvlkbv.net) [84.188.78.135] id=1Eevjx-0001Yn-K5
6001 handling incoming connection from p54bc4e87.dip.t-dialin.net
(yuhvlkbv.net) [84.188.78.135] id=1Eevjx-0001Yn-K5
6001 handling incoming connection from p54bc4e87.dip.t-dialin.net
(yuhvlkbv.net) [84.188.78.135] id=1Eevjx-0001Yn-K5

How can an incoming connection cause such high load? I'm using exim 4.43
and the server handles only mailboxes, no relay.

Regards
Marten