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.

Disadvantages
Once in use, all view counters are wrong forever. If a page is viewed multiple times within total n views on the wiki, the page's view counter is only raised by 1. This leads to incorrect and lower view counts than without using. The higher you set, the more incorrect it will be.