OK. You sync at 5248 so your profile (the top speed you can expect on a good day) is 4500.
Do you ever get more than 2 Meg from a speedtest or is this just particularly bad tonight? Just wondering if you have a stuck profile problem.
Otherwise it's more likely due to contention at this time of night. Cehck the status of your telephone exchange at click here if the staus is RED your exchange is over capacity and would explain slow speeds.
I don't use the TB speedtester anymore as it gives me consistently poor results compared to other testers. Try click here or click here but you may have to log in.
Latency INTERLEAVED is a form of error correction. Your line must have been unstable at some point and throwing up too many errors. The BT monitoring kit must have felt it necessary to switch on interleaving to stabilize the line...nothing to worry about.