Try moving the variables/parameters to the WHERE clause. Having them as part of the join condition may be causing the stored procedure to recompile each time it is called.

update dbo.tMappingRule

set ratio = r.ratio

from dbo.tMappingRule mr

inner join tMappingTemplateRule r

on r.AccountID = mr.AccountID

and r.LevelID = mr.levelID

WHERE mr.DataCycleID = @DataCycleID

and r.MappingTemplateID = @MappingTemplateID