Freeza 3rd form -- 1,850,000
Intial Freeza -- 2,400,000
So about a 1.33x increase to me.
Vegeta would also be tied down with Freeza, at 2,400,000 and then post speed up Freeza would be like 3,200,000.
There're ppl who think Vegeta was even in a losing point vs initial Freeza, as he was seen sweating watching Freeza's performance and was sweating when sensing his initial power, plus counting strongly he was a SSJ as if this belief was like a faith he needed to have to assure his confidence while having a slight to moderate power disadvantage, so I've seen ppl putting initially Freeza holding a similar gap over Vegeta as his 1st form Freeza did over pre Zenkai Vegeta... Which also makes sense considering Vegeta's reactions to 1st form Freeza powering up seems to match his reactions towards initial final form Freeza, tho Vegeta is still close enough he could see and follow Freeza's movements and forced Freeza to increase his speed to be too fast for Vegeta and utterly dominate him. And Vegeta's confidence can't be totally off base too, if he saw Freeza outclassed him by a huge amount, even if he's still sticking to the SSJ feeling, he'd know better there was just nothing he could do. The team work he believed was the key to defeat first form Freeza may be replaced by the belief he was a SSJ now. So something like this works well too:
3rd form Freeza : 1,850,000
Post Zenkai Vegeta : 2,400,000
Initial Final Freeza : 2,650,000
--- post speed up : 3,200,000
So a 70/100 increase, which is huge, and then a 82/100 increase to complement when raising his speed.
I think the 2nd option seems nicer to me.