It depends why it has dropped.
If what caused the drop has also resulted in banding being applied at 18,000kbps then there will be no speed difference in the test socket but the noise margin may be higher.
If it is a wiring problem of some sort on your premises then the test socket will either give you a higher speed, or not - see the previous paragraph.
The only way we can give any better response is with full stats taken in the test socket and with your normal setup, taken as closely together as possible. Not to the fewest possible number of seconds, but preferably within a few minutes. Then we can try to analyse the differences.
Have you at any stage during this decline done anything about it in an attempt to regain speed? Also was it one or two big drops, or a fairly steady decline?
Importantly, are you giving us connection speeds in your post, or speed test results?
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