how to fix "sudo su -" when it has become slow in starting

in case someone is wondering why, from time to time, on a linux server the command 

sudo su -

 seems stuck or just much slower than normal in starting..  this may be because  when the command 

 sudo su -

is starting, by default it scans   wtmp to report last login time from user root and, if on such servers there are hundreds of users, last root access entry may be deep in the access list 

 
the quick trick I have found to fix it is to launch first a simple 

sudo bash 

(so a new entry for root is reported at the top of wtmp) and then closing it at once - after that any next 

sudo su - 

returns to be as fast as usual