Unusably slow sheet / max exceeded errors (suspect Fill Down + max row interaction)

Unusably slow sheet / max exceeded errors (suspect Fill Down + max row interaction)

Hello,

I just started using Zoho Sheets and until I started working with formulas, all was well.

At this point though, my sheet has become unbearably slow. I don't have too many formulas in yet and  under 200 rows of data.

Columns: not sure how to count them in Zoho but they go from A-BZT
Rows: 262144 (around 200 with intended* data)

In short, *I suspect 262k rows + using 'fill down' with formulas is a recipe for disaster. 

Here are the problems, other than slowness and general issues with saving/undo/redo:
1. Problems and delay with saving/undo/redo
2. No way to delete excess columns and rows
3. Can't insert columns and rows because of the "maximum exceeded" error, leaving me to drag empty ones at the end around

I've tried:
1. Deleting extra rows/columns - they just seem to regenerate
2. Locking excess rows/columns (rows after 500, columns after Z) to prevent fill down from affecting them - this prevents moving the columns, which is especially bad when I can't insert any either (see above)

It seems like several of the features are suffering from a lack of safeguards.

So what's the actual best practice? Is there a workflow that will allow me to fill down as quickly but without affecting all the rows? 

Maybe something with tables or named ranges?

Thanks