Thanks AITpro for your input !
Well in fact my thought now, after few tries to reduce all ressources needed to make my wordpress site work, is that all those efforts will not be enough to reduce enough necessary ressources.
So, as Simple history is a very usefull plugin and I can't imagine not to use it and as my host (site5) doesn't seems to be very flexible about ressource (grrrr....) the solution is to change host ! siteground ? any advice ? I can't pay for dedicate server for now.
@Pär Thernström
It's really difficult to put more time on debuging simple history and I even don't know how to find out what are the queries...so if you can be more precise on how to find these infos...