slow macbook

lishmanb wrote:

my macbook pro becomes extremely slow and laggy once the battery reaches 75% or less or or when unplugged after 75% what could this be please ?

You do not say what exact Mac /model /year /size /retina /TouchBar is this

or

what exact macOS you are currently running?

See if you qualify:

13-inch MacBook Pro (non Touch Bar) Battery Replacement Program- October 2016 and October 2017

https://support.apple.com/13inch-macbookpro-battery-replacement

15-inch MacBook Pro Battery Recall Program- September 2015 and February 2017

https://support.apple.com/15-inch-macbook-pro-battery-recall

What is the condition of the battery—hold the Option key and click open the batt icon in the upper right menu bar.

Anything but <Condition : Good >needs to be serviced.