Manual:$wgHitcounterUpdateFreq

Details
Sets how often page counters should be updated, higher values are easier on the database. A value of 1 causes the counters to be updated on every hit, any higher value n cause them to update on average every n hits. Should be set to either 1 or something largish, e.g. 1000, for maximum efficiency.

To disable the page counters entirely, set $wgDisableCounters to true.

Technical details
When this setting is set to a number bigger than 1, the page id of any page viewed is stored in the hitcounter table. About 1 in every ($wgHitcounterUpdateFreq)*0.25 hits, the number of entries in the hit counter table is checked. If there is more than $wgHitcounterUpdateFreq entries in the hitcounter table, then for each entry in the hitcounter table, the relevant page_counter field in the page table is updated by 1 (and the hitcounter table is emptied). Note if the same page is visited twice (or more) between updates of the page table, it is only counted once.