In this case however if its fast path and interleaved, that suggests to me its using G.INP so the stuff on that page doesn't really apply as its different to normal INP and doesn't add latency.
Fast path and interleaved isn't a G.INP thing. ADSL2+ and VDSL can make use of the interleaved channel in this way regardless of g.inp.
It's also possible to see interleave depths of 4,8 on adsl. In theory ADSL has an interleave depth of up to 64 - not that OR or BTw use that high a setting. It's even higher on VDSL depending upon which profile is in use eg 3072 for profile 17a.
VDSL and ADSL2+ often makes use of the interleaved channel to transmit non interleaved data. This leaves the FAST channel for time critical applications such as IPTV, VoIP. Using Interleaved Depth of 1 effectively turns the Interleaved channel into FAST mode. Interleave is a separate process to G.INP in that either or both can be on/off.
All Openreach VDSL lines use the interleaved channel to transmit non-time critical data, the FAST channel being reserved for
IPTV such as BTinternet
Its the level of INP that is usually the clue when it comes to G.INP being enabled.
ADSL2+/VDSL use the INP value to calculate the error protection params based on the line rate.
Openreach retransmission (G.INP) usually has higher INP values somewhere in the 30s'/40s.
It's these higher levels that caused a lot of the problems with Openreach g.inp Mk1 where non g.inp lines were getting INP values of 30+ causing enormous levels of error protection and thus such low sync speeds. It was the high INP value of >30 how I first identified which lines had g.inp when I was responsible for
reporting problematic lines with g.inp to Openreach back in 2015 when G.INP Mk1 was first introduced.
For example on my line, currently
Interleave depth: 1 1
INP: 46.00 0
G.INP: Enabled Not enabled
Interpretation
Data being sent over the interleaved channel depth of 1, so interleaving is in effect off.
INP = 46 meaning high level of INP protection = BTOR retransmission high.
======================
You also have to be careful when someone says they have Interleave 8, as they could either be meaning the depth or the delay.
In this particular case the OP has clarified in the topic title that it is interleave depth.