Thanks for the question. If you are moving multiple Octopus databases to a single server the impact on performance will depend on the utilisation of the current servers vs the new one and the proximity of the current servers vs the new one.
For utilisation you should be able to get an approximate performance impact by monitoring your current database servers. If CPU, memory, disk or network utilisation is close to capacity on your current database servers and you are combining multiple instances then I would expect a negative performance impact. If you have plenty of spare capacity on your current database servers then I would expect minimal performance impact. Alternatively you may be able to adjust the specifications of the new database instances to match the increase in utilisation.
For proximity, if your current database servers are located on a network near the application server and you are moving to a network farther from the application servers I would expect a performance impact. Network latency from the application servers to the database servers should provide a good indication of the impact this will have. For example if you are moving from 10ms to 500ms latency I would expect a significant performance degradation.
Those are the only factors I can think of. If you have enough capacity on the new database servers and low latency from the database servers to the application servers I would expect similar performance to your current experience.
You can also stack multiple application servers if that helps. There is a webinar providing tips to address scaling out Octopus servers happening on the 4th and 5th of December that you may be interested in: https://hello.octopus.com/webinar-spaces-workers
Hope this helps.