For me that delay is about 5 minutes, I’m using the Dexcom G6. If I do a fingerstick in the intervening period I seem to get a lower result, but not as low as my bg eventually goes, so I think the fingerstick is delayed by maybe 2-3 minutes from the level in the blood going to my brain.
For me vision disturbance happens somewhere in the range 30-40mg/dl, certainly under 50. I’ll let a sub-80 figure hover without immediate treatment if the CGM shows a flat line and I’m at home with my wife, but if it hits 70 I will always treat. This is pretty rare these days because I start doing something when I notice my BG heading down below 100 unless it is descending really slowly. I don’t always use the arrow; I prefer looking at the graph. On my phone if I “squeeze” the Dexcom G6 display it overrides the autorotate-lock and gives me a longer horizontal graph which I find easier to read because the base display is only 4 hours; the horizontal display allows 24 hours.
With the graph I can normally guesstimate where the CGM will be in a few minutes. I figure this is a more accurate estimate of where my blood glucose is now than the current CGM reading. The Dexcom prediction of a low doesn’t really work; i.e. the “Urgent Low Soon” setting doesn’t tell me soon enough that something is about to go wrong.
I also use SugarMate, but it’s predictve low alert requires a downward trend of at least an hour. Yesterday I got a down trend that hit 80 in 45 minutes; this was caused by an overbolus which itself seems to have been caused by the G6 reading maybe 40mg/dl high, at least the downwards trend started with what looks like a G6 transmittter reset from about 180mg/dl at 13:00 to 130mg/dl in maybe 10 minutes:
The grey bar on the SugarMate graph covers the range 80-120mg/dl, so Dexcom alerted me when I went below 80 but SugarMate didn’t say anything until later. I started minor corrections at around 14:20 (I didn’t enter that into SugarMate) then bobbed around just below 80 until I ate (then I think I overbolused slightly again!) Ironically this low was also resistant to what I was doing, but then I think that was because I overbolused twice.
Getting the exact data is very difficult on an iPhone with the apps I’m using. I have to get it from Apple Health, but the G6 delays updates to Apple Health by 3 hours, so basically Dexcom lock me out of the record until three hours have elapsed 
Anyway, the data in Apple Health doesn’t quite match the data recorded by SugarMate; the G6 appears to have adjused one point. Previously I thought it was adjusting more, but the I discovered my line up of the two screen shots was off by one measurement. Here is what UnderMyFork shows from the Apple Health data:
Superimposing the SugarMate graph (direct from the Dexcom app) over the SugarMate (from Apple Health data published by the G6 after three hours editorial review) gives this (I just updated this with the correct alignment):
You can see from the full displays that both apps report a “before” bg of 163 and an “after” of 79. The 6 unit bolus was given as 4.35IU immediate plus 1.65IU with a half hour delay. When I line up the right dots the only ones that seem different are the two either side of the sudden drop around 13:20. The change in the Apple Health recoding (UnderMyFork, the colored dots) makes the drop less precipitous.