Power BI - Multiple Reports, One Semantic Model

@scrimpton the size of dataset and refresh duration are very important.
If dataset refresh takes 5-10 minutes, maybe some report will get a little bit slower, but it will just be short peak.
If you have shared capacity (not premium capacity) max size of dataset is 1 GB.
If you have premium capacity, then you can upload datasets bigger than 1 GB. But if you use premium, i doubt that there will be issues if you have 1 dataset and 30 concurrent users.

Where you might get into issues: if you have very complicated dax. Then even small dataset can have huge impact on all reports. 
If you have DAX calculated tables and columns, it will impact on dataset refresh performance.
If you have DAX measures, it will not impact dataset refresh performance, but it will impact report performance.

So best practice:
1) make data model clean and without complex calculations
2) try to refresh during off-peak hours
3) test performance of report using option performance analyzer
4) if you have premium capacity, you can instal Fabric Capacity Metric app where you can see which dataset is slow, at which hours dataset/report gets slow
5) one additional important thing: reports are rendered on user's browser. So if you have fast laptop maybe everything will look fine. But if someone has 4GB ram, slow processor, it will be slow report on user's side for sure if there is a lot of data.

Cheers,
Nemanja Andic