MS Excel VBA workaround for "Slow Conditional Formatting" Issue

It is documented drawback of conditional formatting in MS Excel (2016) that if you have too many formatting cells on-screen at one time, the sheet will become slow or unresponsive. We have such a sheet and we really need to solve this problem for our homecare business in the Midlands of the UK.
I believe I have exhausted all non-coded options for solving the problem through the testing I have done. I would now like someone to propose a graceful solution to the problem using more technical methods, probably VBA coding.
The sheet in question contains a lot of 6-cell boxes which we move around on a "planner". Time axis runs down the left from 0630 to 2230. The top left cell of each box works works out the planned time from the row number. The bottom right cell contains the ideal time for that call. We want the top left cell to change colour based on how far away we are from the ideal time. Screenshot example attached.
We can do this with conditional formatting but when you have 100+ boxes on-screen at the same time, dragging and dropping a single box takes 6 seconds to refresh. We need it to be quicker than this.