I take it you are using the technique of copying the Fplan into the secondary and then lateral revision to change destination etc etc.
On older A320s with legacy FMS with a large flight plan in the primary, it will not be able to copy over the full flight plan into the secondary as well (when you try you get the Fplan full message). In this case build up the secondary from scratch, dont copy the primary into the secondary and modify. I have this, for example, flying an old A320 from Larnaca to Birmingham.
If you activate a secondary flight plan with nothing in it, there will be no performance copied over from the primary and the FMGC computed speeds on the MCDU will be based for the approach on the current FMGC gross weight not the estimated weight at landing. You would have to build up a flight plan and insert perf data.