Hi all, any feedback much appreciated

Batteries undoubtedly age but what I noticed today was I had to keep the key in the ignition position for at least 25 seconds, and could hear starter motor labouring, whirring but slower and slower and slower (thought Id drain battery) X5 then came to life.

Drove about a mile after this, car rested a few hours, on getting back to car and turning key to ignition position it more or less immediately came to life!, there was no slow turnover whirr whirr until it started....so could this be battery throwing out it's last signs of life? or could it be that leaving the key in the ignition/start position for the 25 odd seconds etc could have cleaned or decarboned bushes in starter due to its continued spinning?(yup may well burn out the bushes I guess) Its not the first time I have experienced this, and am 100% sure every time this has happened my starting was more or less instant....the starter motor is audibly quicker.

Temps have dropped here a bit, nothing drastic but enough to give me the slow starts I mention....will give my mechanic a call if he is aware of any issues with the denso started motor I got on the X....was well tempted to buy a new battery too

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E53 X5 3.0d SUV prod.date 11/2004 eng.variant M57N Europe Right hand drive N