Interleaving, and the effect it can have being on or off, is quite complicated. The effect seems to be different on BT Wholesale and LLU networks as well. So as well as my comments below, the actual effect on Plusnet that users report is important. I'm just trying to fill in the background for you.
In theory, on BT Wholesale networks it shouldn't make any difference to connection speed other than in special cases such as the top of ADSL Max. In practice it seems it can do, so my theory knowledge is incomplete.
With regard to throughput, it is important to know, if you don't already, that there is a constant stream of packets between the exchange and your modem when connected, and independent of whether you are using it or not. When not in use the packets are empty of data but still have all the control bits - quite a lot of them!
When you get full speed downloads then the packets are full of data. When there is congestion further back, they are only part full. In both cases there are frequent transmission errors.
Interleaving uses a complex algorithm to correct these errors in flight, you can see how many by the FEC or RS Correction (depending on modem) values. The encoding, decoding and error correction routines are what normally cause a rise in ping/tatency times when it is used.
Without interleaving, any packet that contains even a single-bit error has to re-transmitted. At low error volumes this can be unnoticeable at your end. At high error volumes it can severely affect the throughput, in on top of any congestion issues. Remember, because the packets are always there and always at the connection speed, the amount of data coming down doesn't basically affect the error rate.
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