I assume it is an XPS13. I have one. Slow charge would normally be attributed to the charger/adapter. First, you must verify the charger is putting out the rated voltage (19V). However, the connector does not make that easy to measure without shorting things out. The other option is to get another, but they are valued more than they are worth IMHO.
However, slow charge can also be the battery or the charging circuit - but not usually the latter.
Partial and low charge would be the battery or the charging circuit. Again, not usually the latter.
Of course, since you just replaced it, it might be an installation issue.
Start with the adapter, then re-verify the installation. Then call the vendor for a possible replacement.