After a couple of hours of experimentation I found out that on SAFARI, it's the actual SAVE and RESET functions that are slow. The actual TINT command itself is very fast and works time after time on the same sprite.
I noticed that there is a new Replace RGB command.
Could it be possible to make another similar command, where ALL the colours of the sprite are REPLACED to 1 single colour (white in my case), but BLACK is PRESERVED?
If I had this command I could apply this colorizing to turn a coloured sprite back to WHITE without using SAVE & Reset, and then TINT to a new colour!