I'm wondering what is faster or is there no difference between

for rec in self:
    partner = rec.env['res.partner'].search() # or any other orm method

OR

partner_env = self.env['res.partner']
for rec in self:
    partner = partner_env.search() # or any other orm method

I'm creating a sync with Odoo from other older custom CRM/ERPs and the sync has to go trough 100+, 1000+ or even 10000+ records. So every small performance boost helps.

So what method is faster and why?