Why does selecting/updating on inner-join take so much longer on higher IDs and is threr a way around it?

Question

I have two tables, one with ~250k records one with ~1.5M records.

I am doing a Select and/or Update using Inner Join.

It hung so I decided to to in batches e.g.

Where ID<100000

then

Where ID>=100000 and ID<200000

with each successive batch it takes longer and longer. Adding to the issue of course is there might be records that would have joined w/o being separated by batches.

I'm curious why that is (why do the first 100k IDs join fast and the last 100k take so much longer) and if there is a way to do as a single query w/o the IDs becoming a/the issue (if they in fact are).